Some tests require autogenerated files and do not build for xcore.ai

[oscarbailey-xmos]

Tests affected:

• tests/test_fft_forward
• tests/test_fft_index_bit_reverse
• tests/test_fft_inverse
• tests/test_fft_performance
• tests/test_fft_short_long
• tests/test_fft_split_and_merge

Issues:

• Tests not compiling for xcore-ai, requires a header file "generated.h" generated by xmostest

[oscarbailey-xmos]

These files are generated using gen_tests.py in the tests folder, the only FFT test currently executed for XS2 is test_fft_performance