[java] remove assertj dependency + fix test bugs

[ivmaykov]

There is no good reason to have a dependency on assertj when we already depend on JUnit which has assertions support. This commit does the following:

1. Replaces all assertj assertions with equivalent junit assertions.
2. Fixes several bugs in the tests (such as byte arrays being compared for equality with themselves in Base45Test.java).
3. Removes the assertj dependency.