Replace assert* with more suitable asserts in unit tests

The following replacements were done in unit tests to have
clearer messages in case of failure:
- assertTrue(* is None) with assertIsNone
- assertTrue(* is not None) with assertIsNotNone
- assertTrue(* in *) with assertIn
- assertTrue(* not in *) with assertNotIn
- assertFalse(* in *) with assertNotIn
- assertTrue(* == *) with assertEqual
- assertTrue(* != *) with assertNotEqual

Change-Id: I0c47f991c3974e441335e71c9d26fab8a127f2ca
Merge "Replace assert* with more suitable asserts in unit tests"