This adds 3 tests:
Verify that ChangesDirs() returns nothing when run on a copy of the
same directory.
Verify that various mutations of a directory get reported with the
right changes.
Verify that ExportChanges() + ApplyLayer() of the above mutation gets
an identical directory.
Unfortunately the last test is disabled because it fails in multiple
ways atm. But I want to get it in so that we can fix it.
