Test overwrite_existing apparent symlink dereferencing

To investigate #2006, this explodes the occasionally failing test
`writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed`
into 1000 identical tests, using `test_case::test_matrix`.

When run in series, they all pass, but when run in parallel with
`cargo nextest`, some failures always occur, at least when this is
done on macOS. This is as predicted in:
#2006 (comment)

A more specific result in local testing on macOS 15 is that, while
the number of failures is 0 with `--test-threads=1`, with 2 or more
threads, the number of failures tends to *decrease* with the number
of threads. The most failures occur with `--test-threads=2`, about
30 on the system tested, while `--test-threads=16` has about 10
failures.

The tests were run with this command, with any `--test-threads=<N>`
argument added:

    cargo nextest run -p gix-worktree-state-tests writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed --no-fail-fast

Author: EliahKagan

Cargo.lock

@@ -33,3 +33,4 @@ symlink = "0.1.0"
 once_cell = "1.21.3"
 
 walkdir = "2.3.2"
+test-case = "3.3.1"

gix-worktree-state/tests/Cargo.toml

@@ -12,6 +12,7 @@ use gix_object::{bstr::ByteSlice, Data};
 use gix_testtools::tempfile::TempDir;
 use gix_worktree_state::checkout::Collision;
 use once_cell::sync::Lazy;
+use test_case::test_matrix;
 
 use crate::fixture_path;
 
@@ -103,8 +104,8 @@ fn accidental_writes_through_symlinks_are_prevented_if_overwriting_is_forbidden(
     }
 }
 
-#[test]
-fn writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed() {
+#[test_matrix(0..=999)]
+fn writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed(_i: i32) {
     let mut opts = opts_from_probe();
     // with overwrite mode
     opts.overwrite_existing = true;