Replace `VerifyDir` and `CompareFiles` with
`AssertDirectoryTreesAreEquivalent` and `AssertFilesAreEqual`.
Move `WriteFile` to `TestUtils`.
Add "Symlink" category to start.sh.
This adds a basic symlink policy test that performs backups using each
symlink policy.
If you don't have the required privilege in Windows, you can get an
exception trying to create a symbolic link. If an exception is thrown
trying to create a symbolic link, the exception is trapped and the
test is marked as "Ignored"; e.g.,
```
1) Ignored : Duplicati.UnitTest.SymLinkTests.SymLinkPolicy
Client could not create a symbolic link. Error reported: (1314) A required privilege is not held by the client. | Read: [C:\td\backup-data\target] | Write: [C:\td\backup-data\symlink]
```
