PrefetchVerbTests.PrefetchCleansUpStalePrefetchLock fails intermittently

While refactoring code to light up 'prefetch --hydrate', I noticed that on Windows, this test fail intermittently. I have only seen it fail when the other LockHolder tests are failing (see #176), but often the LockHolder tests will fail without it.

See https://gvfs.visualstudio.com/ci/_build/results?buildId=2617&view=logs for one sample:

2018-10-12T12:36:50.1053529Z Errors, Failures and Warnings
2018-10-12T12:36:50.1060127Z 
2018-10-12T12:36:50.1060828Z 1) Failed : GVFS.FunctionalTests.Tests.EnlistmentPerFixture.GitReadAndGitLockTests.GitAliasNamedAfterKnownCommandAcquiresLock()
2018-10-12T12:36:50.1061496Z   Expected substring 'Waiting for 'GVFS.FunctionalTests.LockHolder'' not found in ''
2018-10-12T12:36:50.1061932Z   Expected: True
2018-10-12T12:36:50.1062282Z   But was:  False
2018-10-12T12:36:50.1062896Z    at GVFS.Tests.Should.StringShouldExtensions.ShouldContain(String actualValue, String[] expectedSubstrings) in C:\agent\_work\2\s\GVFS\GVFS.Tests\Should\StringShouldExtensions.cs:line 19
2018-10-12T12:36:50.1064058Z    at GVFS.FunctionalTests.Tests.EnlistmentPerFixture.GitReadAndGitLockTests.GitAliasNamedAfterKnownCommandAcquiresLock() in C:\agent\_work\2\s\GVFS\GVFS.FunctionalTests\Tests\EnlistmentPerFixture\GitReadAndGitLockTests.cs:line 71
2018-10-12T12:36:50.1064701Z 
2018-10-12T12:36:50.1065616Z 2) Failed : GVFS.FunctionalTests.Tests.EnlistmentPerFixture.GitReadAndGitLockTests.GitAliasInSubfolderNamedAfterKnownCommandAcquiresLock()
2018-10-12T12:36:50.1066354Z   Expected substring 'Waiting for 'GVFS.FunctionalTests.LockHolder'' not found in 'error: Failed to merge in the changes.
2018-10-12T12:36:50.1066862Z '
2018-10-12T12:36:50.1067173Z   Expected: True
2018-10-12T12:36:50.1067573Z   But was:  False
2018-10-12T12:36:50.1068218Z    at GVFS.Tests.Should.StringShouldExtensions.ShouldContain(String actualValue, String[] expectedSubstrings) in C:\agent\_work\2\s\GVFS\GVFS.Tests\Should\StringShouldExtensions.cs:line 19
2018-10-12T12:36:50.1069507Z    at GVFS.FunctionalTests.Tests.EnlistmentPerFixture.GitReadAndGitLockTests.GitAliasInSubfolderNamedAfterKnownCommandAcquiresLock() in C:\agent\_work\2\s\GVFS\GVFS.FunctionalTests\Tests\EnlistmentPerFixture\GitReadAndGitLockTests.cs:line 87
2018-10-12T12:36:50.1070032Z 
2018-10-12T12:36:50.1070496Z 3) Failed : GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PrefetchCleansUpStalePrefetchLock()
2018-10-12T12:36:50.1070925Z   Expected: 0
2018-10-12T12:36:50.1071238Z   But was:  128
2018-10-12T12:36:50.1071798Z    at GVFS.Tests.Should.ValueShouldExtensions.ShouldEqual[T](T actualValue, T expectedValue, String message) in C:\agent\_work\2\s\GVFS\GVFS.Tests\Should\ValueShouldExtensions.cs:line 34
2018-10-12T12:36:50.1072635Z    at GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PostFetchJobShouldComplete() in C:\agent\_work\2\s\GVFS\GVFS.FunctionalTests\Tests\EnlistmentPerFixture\PrefetchVerbTests.cs:line 143
2018-10-12T12:36:50.1073499Z    at GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PrefetchCleansUpStalePrefetchLock() in C:\agent\_work\2\s\GVFS\GVFS.FunctionalTests\Tests\EnlistmentPerFixture\PrefetchVerbTests.cs:line 113