Fix flaky WriteWithoutClose/CreateFileWithoutClose functional tests

[tyrielv]

Summary

Fix GC-dependent race condition causing flaky failures in StatusTests.WriteWithoutClose() and StatusTests.CreateFileWithoutClose().

Problem

These tests intentionally open file handles without closing them to validate that git status works with locked files. The handles were created as local variables inside OpenFileAndWriteWithoutClose/CreateFileWithoutClose with no reference kept — making them eligible for GC immediately. If the GC finalized the handles before TearDown ran git reset --hard, the reset would succeed (no lock errors) while the control repo might still have errors, causing a flaky error mismatch in TearDown.

Fix

• Change FileSystemRunner.CreateFileWithoutClose and OpenFileAndWriteWithoutClose to return IDisposable instead of void
• SystemIORunner returns the actual FileStream/StreamWriter handle, with an explicit Flush() to ensure data is on disk
• GitRepoTests helpers return a CompositeDisposable wrapping handles for both GVFS and control repos
• StatusTests uses using blocks — handles stay alive during the assertion scope and are deterministically disposed before TearDown runs

This eliminates the GC race: both repos have unlocked files when reset --hard runs in TearDown, so errors match (both 0).

Testing

• Built GVFS.FunctionalTests project successfully (C# compilation clean)
• The fix is structural — deterministic disposal replaces non-deterministic GC finalization

[dscho]

Nice!