Consistent usage of TestNG

[frantuma]

Includes and replaces #4879 (fixes gradle test), @strangelookingnerd thanks!

This PR aims to remove usage of JUnit and consistently use TestNG across the modules instead.

Changes include:

• Use TestNG annotations over JUnit counterparts
• Use TestNG assertions over JUnit counterparts (also simplify some assertions in the process)
• Consistently use static imports for TestNG assertions
• Remove JUnit depencency if not required
• Add TestNG dependency if required

There is one exception in swagger-maven-plugin where JUnit is still required by the underlying maven test suite and thus tests can not be migrated.