Test fewer test parallelism values and reps

- Test `--test-threads` operands of 1 to 4, rather than 1 to 7.
- Do 7 reps instead of 16.

In addition to the 1000x repetition in `checkout.rs` of the
writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed
test case, which in practice always surfaces GitoxideLabs#2006 on CI on macOS
and Windows, the `test-ext4-casefold` job as it currenly stands
seems sufficient to test whether a possible future solution would
fix the bug on GNU/Linux.

This looks like about the minmum amount we should reasonably verify
still fails (in some jobs) immediately before a GitoxideLabs#2006 fix, while no
longer failing *any* jobs afterwards. But the `test-ext4-casefold`
jobs are still much more than would be wanted on the main branch.

Assuming the cause of the failures is the same on all systems, and
the solution neither explicitly nor implicitly operates differently
across systems (when comparing how it works when the filesystem is
case-insensitive, that is), it may be enough on CI to regression
test only macOS and Windows for GitoxideLabs#2006.

Thus, this also adds a TODO comment atop the `test-ext4-casefold`
definition reminding that it should not be kept, or not in full.

Author: EliahKagan

.github/workflows/ci.yml

@@ -185,20 +185,18 @@ jobs:
       - name: Check that tracked archives are up to date
         run: git diff --exit-code  # If this fails, the fix is usually to commit a regenerated archive.
 
+  # TODO(integration): Remove all or at least most test-ext4-casefold jobs prior to merging #2008.
   test-ext4-casefold:
     strategy:
       matrix:
         # `--test-threads` operand
         parallel-tests:
-          - 1  # Never seems to fail, which makes sense.
-          - 2
-          - 3  # Never seems to fail, which does NOT make sense.
-          - 4
-          - 5
-          - 6
-          - 7
+          - 1  # Never fails.
+          - 2  # Usually fails.
+          - 3  # Almost never fails, somehow!
+          - 4  # Usually fails.
         # Duplicate jobs
-        rep: ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', O, P]
+        rep: [A, B, C, D, E, F, G]
       fail-fast: false
 
     runs-on: ubuntu-latest