Modify the test code to use single-threaded checkout

This moves setting `thread_limit: Some(1)` out of being a step done
with `sed` in the `test-ext4-casefold` jobs, and into being an
actual change to the source code of the `checkout.rs` test module.

This change is intended to be temporary. The goal is the same as
before, but to observe the effect outside `text-ext4-casefold`.

That is, the goal here is to temporarily see if there is a change
in results in the other jobs where failure almost always occurs
due to GitoxideLabs#2006, i.e., the `test-fast` jobs on `macos-latest` and
`windows-latest`, and the `test-fixtures-windows` job.

Note that, as of this change, the `gix-features/parallel` feature
is only turned off in the `test-ext4-casefold` jobs. In others,
that feature is still turned on, just a parallelism of 1, i.e., a
single thread, is used for the checkout. But a parallelism of 1
seems usually to be special-cased to not use facilities related to
multithreading.

(This `thread_limit` argument change, as well as the immediately
preceding change of disabling the `parallel` feature flag in the
`test-ext4-casefold` test jobs, are entirely separate from how many
writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed
duplicated test cases there are, and also entirely separate from
the operand to `--test-threads`, which actually controls the number
of test *processes* `cargo nextest` creates to run the tests.)

Author: EliahKagan

.github/workflows/ci.yml

@@ -247,11 +247,6 @@ jobs:
           # Prepend leading digits "24" to the upper bound of the label range.
           sed -Ei 's/^(#\[test_matrix\(0\.\.=)([[:digit:]]+\)])$/\124\2/' \
             gix-worktree-state/tests/state/checkout.rs
-      - name: Modify the test code to use single-threaded checkout
-        run: |
-          # Replace the usual `thread_limit` argument with `Some(1)`.
-          sed -i 's/gix_features::parallel::num_threads(None).into()/Some(1)/' \
-            gix-worktree-state/tests/state/checkout.rs
       - name: Display the changes made for this test
         run: git diff
       - name: Test writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed (nextest)

gix-worktree-state/tests/state/checkout.rs

@@ -689,10 +689,11 @@ fn probe_gitoxide_dir() -> crate::Result<gix_fs::Capabilities> {
 fn opts_from_probe() -> gix_worktree_state::checkout::Options {
     static CAPABILITIES: Lazy<gix_fs::Capabilities> = Lazy::new(|| probe_gitoxide_dir().unwrap());
 
+    // FIXME(integration): Restore multithreaded `thread_limit` prior to merging #2008.
     gix_worktree_state::checkout::Options {
         fs: *CAPABILITIES,
         destination_is_initially_empty: true,
-        thread_limit: gix_features::parallel::num_threads(None).into(),
+        thread_limit: Some(1), // gix_features::parallel::num_threads(None).into(),
         ..Default::default()
     }
 }