Migrate tests to JUnit5

[strangelookingnerd]

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

• Migrate annotations and imports
• Migrate assertions
• 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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[MarkEWaite]

I was surprised to see that test coverage decreased with this change as reported by the "Indirect changes" page of https://ci.jenkins.io/job/Plugins/job/github-oauth-plugin/job/PR-290/1/coverage/ . The GithubAuthenticationToken class reduced its statement coverage and its branch coverage.

Is there a compelling reason that we can't retain comparable coverage with JUnit 5 as we had with JUnit 4?

[strangelookingnerd]

I was surprised to see that test coverage decreased with this change as reported by the "Indirect changes" page of https://ci.jenkins.io/job/Plugins/job/github-oauth-plugin/job/PR-290/1/coverage/ . The GithubAuthenticationToken class reduced its statement coverage and its branch coverage.

Is there a compelling reason that we can't retain comparable coverage with JUnit 5 as we had with JUnit 4?

I double checked my changes and found two additional assertions that were checking the contract of equals that I seem to have removed by accident. With these back in place the Coverage has improved a little but is still short compared to master.

I double checked the jacoco reports locally and found some oddities: According to the Jacoco Report for JUnit5 GithubAuthenticationToken#clearCaches() is not covered at all (= never called). Which I can not confirm, as it is definitly called multiple times during tests. Same goes for #getGithubServer() and some other methods / fields.

I could imagine that switching to MockitoExtension over MockitoJUnitRunner could be involved here, but I would not know how to verify that.

Edit: Ignore this...

[strangelookingnerd]

I gave it another go and found the culprit to be in GithubRequireOrganizationMembershipACLTest. I accidentally changed one of the mocks which caused the overall coverage to decrease. I fixed it now. Coverage is back to normal. Sorry for the fuss.

[MarkEWaite]

Thanks!