Run test suite without ffmpeg dependencies

[ericpre]

Since ffmpeg seems to be an optional dependency (?), the test suite should be run without failure when ffmpeg is not installed.

From https://github.com/hyperspy/hyperspy-extensions-list/actions/runs/12596586494/job/35107968824?pr=64

============================= test session starts ==============================
platform linux -- Python 3.12.8, pytest-8.3.4, pluggy-1.5.0
Matplotlib: 3.10.0
Freetype: 2.[12](https://github.com/hyperspy/hyperspy-extensions-list/actions/runs/12596586494/job/35107968824?pr=64#step:29:13).1
rootdir: /home/runner/work/hyperspy-extensions-list/hyperspy-extensions-list
plugins: mpl-0.17.0, rerunfailures-[15](https://github.com/hyperspy/hyperspy-extensions-list/actions/runs/12596586494/job/35107968824?pr=64#step:29:16).0, xdist-3.6.1
created: 2/2 workers
2 workers [262 items]

..........FFF.........................ss................................ [ 27%]
....................................................................ssss [ 54%]
ssssssss................................................................ [ 82%]
.s............................................                           [100%]
=================================== FAILURES ===================================
____________________ TestCommonStack.test_save_movie[axis0] ____________________
[gw1] linux -- Python 3.12.8 /home/runner/miniconda3/envs/test/bin/python

self = <etspy.tests.test_base.TestCommonStack object at 0x7f2beb99b6e0>
tmp_path = PosixPath('/tmp/pytest-of-runner/pytest-4/popen-gw1/test_save_movie_axis0_0')
axis = ('XY', 77)

    @pytest.mark.parametrize("axis", [("XY", 77), ("XZ", 256), ("YZ", 256)])
    def test_save_movie(self, tmp_path, axis):
        s = ds.get_needle_data(aligned=True)
        f = tmp_path / f"output_movie_{axis[0]}.avi"
>       s.save_movie(start=0, stop=axis[1], outfile=f, axis=axis[0])

../../../miniconda3/envs/test/lib/python3.12/site-packages/etspy/tests/test_base.py:88: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../../../miniconda3/envs/test/lib/python3.12/site-packages/etspy/base.py:524: in save_movie
    writer = animation.writers["ffmpeg"](fps=fps)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <matplotlib.animation.MovieWriterRegistry object at 0x7f2bebb3f1a0>
name = 'ffmpeg'

    def __getitem__(self, name):
        """Get an available writer class from its name."""
        if self.is_available(name):
            return self._registered[name]
    writer = animation.writers["ffmpeg"](fps=fps)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <matplotlib.animation.MovieWriterRegistry object at 0x7f2bebb3f1a0>
name = 'ffmpeg'

    def __getitem__(self, name):
        """Get an available writer class from its name."""
        if self.is_available(name):
            return self._registered[name]
>       raise RuntimeError(f"Requested MovieWriter ({name}) not available")
E       RuntimeError: Requested MovieWriter (ffmpeg) not available

../../../miniconda3/envs/test/lib/python3.12/site-packages/matplotlib/animation.py:1[21](https://github.com/hyperspy/hyperspy-extensions-list/actions/runs/12596586494/job/35107968824?pr=64#step:29:22): RuntimeError
=============================== warnings summary ===============================
../../../miniconda3/envs/test/lib/python3.12/site-packages/traits/observation/_generated_parser.py:258
../../../miniconda3/envs/test/lib/python3.12/site-packages/traits/observation/_generated_parser.py:258
  /home/runner/miniconda3/envs/test/lib/python3.12/site-packages/traits/observation/_generated_parser.py:258: DeprecationWarning: module 'sre_parse' is deprecated
    import sre_parse

../../../miniconda3/envs/test/lib/python3.12/site-packages/traits/observation/_generated_parser.py:259
../../../miniconda3/envs/test/lib/python3.12/site-packages/traits/observation/_generated_parser.py:259
  /home/runner/miniconda3/envs/test/lib/python3.12/site-packages/traits/observation/_generated_parser.py:259: DeprecationWarning: module 'sre_constants' is deprecated
    import sre_constants

tests/test_base.py::TestCommonStack::test_save_movie[axis1]
tests/test_base.py::TestCommonStack::test_save_movie[axis2]
tests/test_base.py::TestCommonStack::test_save_movie_invalid_axis
  /home/runner/miniconda3/envs/test/lib/python3.12/site-packages/matplotlib/animation.py:908: UserWarning: Animation was deleted without rendering anything. This is most likely not intended. To prevent deletion, assign the Animation to a variable, e.g. `anim`, that exists until you output the Animation using `plt.show()` or `anim.save()`.
    warnings.warn(

tests/test_utils.py::TestMultiframeAverage::test_register_serialem_stack_multicpu
tests/test_utils.py::TestMultiframeAverage::test_register_serialem_stack_multicpu
  /home/runner/miniconda3/envs/test/lib/python3.12/multiprocessing/popen_fork.py:66: DeprecationWarning: This process (pid=5035) is multi-threaded, use of fork() may lead to deadlocks in the child.
    self.pid = os.fork()

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=========================== short test summary info ============================
FAILED tests/test_base.py::TestCommonStack::test_save_movie[axis0] - RuntimeError: Requested MovieWriter (ffmpeg) not available
FAILED tests/test_base.py::TestCommonStack::test_save_movie[axis1] - RuntimeError: Requested MovieWriter (ffmpeg) not available
FAILED tests/test_base.py::TestCommonStack::test_save_movie[axis2] - RuntimeError: Requested MovieWriter (ffmpeg) not available
============ 3 failed, [24](https://github.com/hyperspy/hyperspy-extensions-list/actions/runs/12596586494/job/35107968824?pr=64#step:29:25)4 passed, 15 skipped, 9 warnings in 55.00s ============