Mock SelectorsHandler in Element tests to break dependency on NamedSelector

What was done:

1. common NamedSelector mocking code moved out to special `mockNamedFinder` method
2. added additional mocking code for direct xpath matching cases
3. common test case code moved to parent `ElementTest` class

Native PHPUnit mocking library used in all cases.

Fixes #436.

__Once merged, the #438 needs to be rebased on to this to make test pass.__