Replacing dots with slashes in tag/category names.
This is because it messes up the code that figures out what extension to use
for an output file during the bake.
Better unit test infrastructure:
- Fixed inconsistent namespaces for test cases.
- All `MockFileSystem` instances that use real paths now use a predefined
  directory in the `tmp/mock` directory.
- Added base test case class that cleans up the `tmp/mock` directory. Removed
  old way to clean up.