Fix flaky ModifiedPathsTests by replaying the on-disk log

[tyrielv]

Summary

Fixes the flaky ModifiedPathsTests.DeletedTempFileIsRemovedFromModifiedFiles test (and any other modified-paths assertion that hits the same race).

System.InvalidOperationException : Sequence contains more than one matching element
   at GVFS.Tests.Should.EnumerableShouldExtensions.ShouldNotContain[T](IEnumerable`1, Func`2)
   at GVFS.FunctionalTests.Tools.GVFSHelpers.ModifiedPathsShouldNotContain(...)

Root cause

ModifiedPathsDatabase is an append-only A/D log that gets compacted at the end of each background-op batch via WriteAllEntriesAndFlush in PostBackgroundOperation. BackgroundOperationCount drops to 0 inside DequeueAndFlush for the last task — before the post-callback (and therefore the compaction) runs. WaitForBackgroundOperations polls until count==0 and can return between those two steps, observing a transient state like:

A temp.txt
D temp.txt

The old ModifiedPathsShouldNotContain matched both lines, fed two results into EnumerableShouldExtensions.ShouldNotContain, and SingleOrDefault threw InvalidOperationException — masking the real race with a confusing exception.

Product code is correct

FileBasedCollection.TryLoadFromDisk replays the A/D log on mount and only the final state reaches the in-memory set, so the on-disk file is always recoverable. The append-then-amortized-rewrite pattern is intentional (one O(1) write per FS event vs. O(N) on every op). No product change needed. The new AddFollowedByDeleteIsRecoveredOnLoad unit test pins down this recovery contract explicitly.

Changes (all test-only)

• GVFSHelpers.ModifiedPathsShouldContain / ModifiedPathsShouldNotContain — Replay the A/D log into a HashSet<string> (same algorithm as TryLoadFromDisk) and check semantic membership. Matches the intent of all ~110 existing callers and is robust to the flush-race window.
• GVFSHelpers.ModifiedPathsContentsShouldEqual → ModifiedPathsRawFileContentsShouldEqual — Renamed and documented to make it obvious this asserts on the raw compacted file layout, not the semantic set. Two callers in CheckoutTests.cs updated.
• EnumerableShouldExtensions.ShouldNotContain<T>(predicate) — Uses Where(predicate) instead of SingleOrDefault(predicate) so multi-match cases yield a useful Assert.Fail message instead of InvalidOperationException.
• ModifiedPathsDatabaseTests.AddFollowedByDeleteIsRecoveredOnLoad — New unit test asserting that loading "A temp.txt\r\nD temp.txt\r\n" produces an empty set (only the default .gitattributes).

Verification

• ✅ Full GVFS.UnitTests suite passes (819/819) on Windows.
• ✅ New AddFollowedByDeleteIsRecoveredOnLoad test passes.
• ✅ GVFS.FunctionalTests.csproj builds clean.