[PartitionAlloc] Skip FutexMigration test without PI futex support

The FutexMigration unit test enables PI-futex migration directly, but the runtime only does that on kernels that support PI futexes. Skip the test when that support is absent so unsupported Android kernels don't fail or hang.

Bug: 500418107
Change-Id: Ibf5e89088d33a82b8f74e7cbe4ee7464b7d4f759
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7745842
Reviewed-by: Anand Ravi <anandrv@google.com>
Commit-Queue: Dailen King <dailenking@microsoft.com>
Reviewed-by: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1613822}
NOKEYCHECK=True
GitOrigin-RevId: 04c4c9b00af4ab297a628e94a464b6c0f662c612