Replace `VerifyDir` and `CompareFiles` with
`AssertDirectoryTreesAreEquivalent` and `AssertFilesAreEqual`.
Move `WriteFile` to `TestUtils`.
Add "Symlink" category to start.sh.
The RecoveryTool sets the currenty directory during its operation. We
must reset the current directory so that the test teardown methods do
not complain that paths are being used by other processes.