Be consistent with cleaning up functional tests on Windows and macOS

We recently hit an issue where we had failing tests on Windows due to a second remote being used as the repo under test. This creates a separate .gvfsCache folder and mapping.dat entry.

On Windows, we don't clean up the Functional Tests folder between runs and since a second URL was used, tests that ensure we only have one .gvfsCache fail:

Errors, Failures and Warnings

1) Failed : GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PrefetchCleansUpStalePrefetchLock()
  C:\Repos\GVFSFunctionalTests\enlistment\..\.gvfsCache is expected to have only one folder. Actual folders: C:\Repos\GVFSFunctionalTests\.gvfsCache\29988cba39514a77bf35068f41600402,C:\Repos\GVFSFunctionalTests\.gvfsCache\b810a5d8b978423bb9fe3aa140499e0e
  Expected: 1
  But was:  2
   at GVFS.Tests.Should.ValueShouldExtensions.ShouldEqual[T](T actualValue, T expectedValue, String message) in C:\agent\_work\11\s\GVFS\GVFS.Tests\Should\ValueShouldExtensions.cs:line 34
   at GVFS.FunctionalTests.Tools.GVFSFunctionalTestEnlistment.GetObjectRoot(FileSystemRunner fileSystem) in C:\agent\_work\11\s\GVFS\GVFS.FunctionalTests\Tools\GVFSFunctionalTestEnlistment.cs:line 132
   at GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PostFetchStepShouldComplete() in C:\agent\_work\11\s\GVFS\GVFS.FunctionalTests\Tests\EnlistmentPerFixture\PrefetchVerbTests.cs:line 227
   at GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PrefetchCleansUpStalePrefetchLock() in C:\agent\_work\11\s\GVFS\GVFS.FunctionalTests\Tests\EnlistmentPerFixture\PrefetchVerbTests.cs:line 122

2) Failed : GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PrefetchCleansUpPackDir()
  C:\Repos\GVFSFunctionalTests\enlistment\..\.gvfsCache is expected to have only one folder. Actual folders: C:\Repos\GVFSFunctionalTests\.gvfsCache\29988cba39514a77bf35068f41600402,C:\Repos\GVFSFunctionalTests\.gvfsCache\b810a5d8b978423bb9fe3aa140499e0e
  Expected: 1
  But was:  2
   at GVFS.Tests.Should.ValueShouldExtensions.ShouldEqual[T](T actualValue, T expectedValue, String message) in C:\agent\_work\11\s\GVFS\GVFS.Tests\Should\ValueShouldExtensions.cs:line 34
   at GVFS.FunctionalTests.Tools.GVFSFunctionalTestEnlistment.GetObjectRoot(FileSystemRunner fileSystem) in C:\agent\_work\11\s\GVFS\GVFS.FunctionalTests\Tools\GVFSFunctionalTestEnlistment.cs:line 132
   at GVFS.FunctionalTests.Tools.GVFSFunctionalTestEnlistment.GetPackRoot(FileSystemRunner fileSystem) in C:\agent\_work\11\s\GVFS\GVFS.FunctionalTests\Tools\GVFSFunctionalTestEnlistment.cs:line 141
   at GVFS.FunctionalTests.Tests.EnlistmentPerFixture.PrefetchVerbTests.PrefetchCleansUpPackDir() in C:\agent\_work\11\s\GVFS\GVFS.FunctionalTests\Tests\EnlistmentPerFixture\PrefetchVerbTests.cs:line 144

On macOS, we delete that folder between runs due to issues we encountered when first standing up the Functional Tests. Recent work on stress testing the kext by running the Functional Tests shows that we don't need to delete that folder between runs.

We should be consistent between platforms; either we delete the FunctionalTests folder on both or not.

Start with GVFSFunctionalTestEnlistment.cs and the failing PrefetchVerbTests.cs cases to understand how the Functional Tests folder and .gvfsCache are handled. Compare the Windows and macOS cleanup behavior, then run the affected functional tests on both platforms. Done means both platforms consistently follow the chosen cleanup policy without stale cache failures.