[slachiewicz]

Part of #1414. Thirteen test classes in versions-maven-plugin use plain JUnit 4 and now use JUnit 5; the module gets junit-jupiter-api like versions-common has. None of them passes a message as the first argument of an assertion, so the argument order is unchanged.

The same classes no longer use Hamcrest. equalTo, is, instanceOf, nullValue and empty become the matching Assertions methods. The text matchers have no JUnit counterpart, so TextAssertions adds assertContains, assertNotContains, assertContainsInOrder, assertMatches (the whole text, as matchesPattern did) and assertAnyLineContains, each printing the text it checked when it fails.

The other 27 JUnit 4 classes use AbstractMojoTestCase and MojoRule, the deprecated classes CI warns about, and keep JUnit 4 and Hamcrest for now: the JUnit 5 MojoExtension of the harness gives the mojo an empty MavenProject instead of building the POM, so those tests need a helper that builds the project first. That comes next under #1414, and the vintage engine and Hamcrest go once they move.

Verified: mvn -pl versions-maven-plugin install -Dinvoker.skip → 297 tests, the same count as on master, 0 failures, 0 Checkstyle violations; changing an expected value for assertContains, assertNotContains and assertContainsInOrder makes each fail with the checked text