Merge two unittest modules that test same module

For module 'manila.share.manager' exist two test suites located in
manila.tests.test_share and manila.tests.share.test_manager
This commit merges two mentioned test modules to second one, because it is
located properly.

Changes:
- updated Fake driver in 'manila.tests.fake_driver' module to satisfy
  proper driver interface.
- used updated fake driver in result test suite instead of own implementation.
- moved unittests to another module.
- deleted one overlapping test.

This commit does not add new tests, there are still uncovered functionality
exist.

Change-Id: I7c102e1331f267ac43bcb09c4d9fe9426a0d8853
Partial-Bug: #1348633