Migrate the test suite from JUnit 4 to JUnit 5

[slachiewicz]

Moves 18 test classes to Jupiter. The bulk is the OpenRewrite JUnit4to5Migration recipe, but three things needed hand work — the third is the one worth reviewing.

TransformerTesterRule was a custom TestRule. The recipe converted the @Test annotations in PropertiesTransformerTest to Jupiter but left the @Rule field alone. That still compiles, which is the dangerous part: under Jupiter the rule would simply never run, and the four spec-driven tests would have passed while asserting nothing. It is rewritten as an InvocationInterceptor and applied with @ExtendWith instead of a @Rule field.

One behavioural note on that rewrite: the JUnit 4 rule deliberately never called base.evaluate() when a @TransformerTest spec was present — the test body never ran. Jupiter requires invocation.proceed() to be called exactly once, so the interceptor now does. That is safe here only because all four @TransformerTest methods have empty bodies ({}), and PropertiesTransformerTest is the only class using the annotation. Worth knowing if anyone later adds a body to one of them.

MinijarFilterTest — the recipe emitted non-compiling code, turning TemporaryFolder.builder().assureDeletion().build() into File.builder().... Replaced with a plain @TempDir File field.

DefaultShaderTest — @ClassRule TemporaryFolder TEMPORARY_FOLDER became @TempDir public static File TEMPORARY_FOLDER, which trips two checkstyle rules at once: VisibilityModifier exempts @Rule/@ClassRule but not @TempDir, and StaticVariableName wants lowerCamelCase for non-final statics. Renamed to a package-private static File temporaryFolder.

Left on JUnit 4 deliberately: ShadeMojoTest. It extends AbstractMojoTestCase → PlexusTestCase → junit.framework.TestCase, with methods discovered by the test* naming convention. That inheritance lives in maven-plugin-testing-harness, not here. junit:junit therefore stays (the harness marks it optional, so it must be declared explicitly), and junit-vintage-engine is added so the class stays discoverable once surefire switches to the platform provider.

org.hamcrest:hamcrest also stays — several tests use CoreMatchers/MatcherAssert directly. junit-jupiter-engine is deliberately not declared, since surefire provisions it.

src/it and src/test/resources are untouched — confirmed by diffing the OpenRewrite output so that only pom.xml and src/test/java differ.

Verification

mvn test before: Tests run: 72, Failures: 0, Errors: 0, Skipped: 0 (surefire selecting JUnit4Provider).
mvn test after: Tests run: 72, Failures: 0, Errors: 0, Skipped: 0 (surefire selecting JUnitPlatformProvider, with ShadeMojoTest's 5 tests running via the vintage engine).
checkstyle:check 0 violations; spotless:check clean.

Because "the tests still pass" proves nothing about an extension that might silently do nothing, the interceptor was also checked negatively: corrupting one expected value in PropertiesTransformerTest produces a real AssertionFailedError, so it is genuinely enforcing.

Draft until CI confirms.

Generated-by: Claude Opus 5 (1M context)