Migrate tests to JUnit5

[strangelookingnerd]

This PR aims to migrate all tests to JUnit5. Changes include:

• Migrate annotations and imports
• Migrate assertions
• Migrate GHMock from Rule to Extension
• Migrate DataProviderRunner to ParameterizedTest
• Remove public visibility for test classes and methods
• Minor clean up

I am well aware that this is a quite large changeset however I hope that there is still interest in this PR and it will be reviewed.
If there are any questions, please do not hesitate to ping me.

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[KostyaSha]

for which there is no JUnit5 equivalent

That's why junit5 was not used. What is the goal at all of doing this?

[strangelookingnerd]

That's why junit5 was not used.

As I wrote its not available yet, but will be once jenkinsci/jenkins-test-harness#900 is being delivered via a newer plugin-pom version.

What is the goal at all of doing this?

In general I'd say it's a good practice to not rely on outdated libraries - even for testing. This PR removes one of these outdated libraries (com.tngtech.java:junit-dataprovider) entirely and updates another one to a more recent and therefore supported and maintained version (com.github.tomakehurst:wiremock-jre8-standalone).
Upgrading from JUnit4 to 5 not only brings some advantages in usage and features but also makes the test implementations not depend on a framework version that is no longer maintained.