Change assertTrue(A in B) to assertIn(A, B)

The code in tests was using assertTrue(A in B) and similar
instead of assertIn / assertNotIn.

assertIn / assertNotIn assertions are provided by `testtools` package,
so this change is not breaking backward compatibility with Python 2.6.

Closes-Bug: #1259295

Change-Id: Ibf3b0c7f27895699db8a13bf6e1640d196f1a953
Merge "Change assertTrue(A in B) to assertIn(A, B)"