Consistent usage of TestNG

[strangelookingnerd]

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.

[strangelookingnerd]

I can not reproduce the build / test failures using JDK 11 or 17 reported by the actions using:

./mvnw --no-transfer-progress -B install --file pom.xml
cd ./modules/swagger-gradle-plugin
./gradlew build --info

Am I missing something here?

[frantuma]

Thanks! Included and replaced by #4885