Ban JUnit 4 imports

[strangelookingnerd]

Ban JUnit 4 imports

To prevent regressions when adding new tests, jenkinsci/plugin-pom#1178 introduced a new flag that enables a Maven Enforcer rule banning org.junit.* imports while allowing org.junit.jupiter.*.

With this change, the build will fail if any org.junit.* imports are introduced.

Testing done

None. Rely on ci.jenkins.io.

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[jglick]

I do not wish to use a fork from jenkinsci/jenkins-test-harness#988. I maintain RealJenkinsRule.

[strangelookingnerd]

I do not wish to use a fork from jenkinsci/jenkins-test-harness#988. I maintain RealJenkinsRule.

I'm having a hard time to understand why this is an issue for this PR in particular. I'm almost certain you have reviewed / merged many other of my PR that proposed similar migrations.

If it helps, having the RealJenkinsExtension in use like it is proposed here would most likely still work when it becomes a thin wrapper as you suggested.

[jglick]

why this is an issue for this PR in particular

Just because I saw

structs-plugin/src/test/java/org/jenkinsci/plugins/structs/SymbolLookupAnticacheTest.java

Line 28 in c4dca15

var plugin = rjr.createSyntheticPlugin(new RealJenkinsExtension.SyntheticPlugin(TrivialBuilder.class).shortName("SymbolLookupAnticacheTest").header("Plugin-Dependencies", "structs:0"));

and realized for the first time that the entire block of RJR logic had been copied.