Refactor tests

[mertserezli]

Statements like MatcherAssert.assertThat can be changed to assertThat by using import static to improve readability and reduce line width.

[Vatavuk]

@mertserezli If we use assertThat without accompanied class a user will lose visibility from where this method comes from. It also introduces additional complexity if we want to use a method with the same name through some other third-party library. Although is more verbose, I think using class prefix is cleaner.