Excavator: Switch to JUnit 5 to parallelize tests and speed up CI

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[aldexis]

As discussed internally, [CompilationRule](https://github.com/google/compile-testing/blob/main/src/main/java/com/google/testing/compile/CompilationRule.java) does not have a JUnit5 replacement (per google/compile-testing#146).

There were three options:

• Copy over CompilationExtension from Add JUnit5 implementation of CompilationRule google/compile-testing#155
  • Downside: This is a fairly complex class and requires us to maintain it
• Wrap all the tests in a helper method that does something like

Compiler.javac()
    .withProcessors(new AbstractProcessor() {
        @Override
        public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
            test.accept(processingEnv.getElementUtils());
            return false;
        }
    })
    .compile(DUMMY)

• Downside: we need to update and add unsavory indentation to all these tests
• Leave these tests to use junit4 for now

We decided to go with that third option for now and disable the excavator once this PR merges

[pkoenig10]

Use AssertJ

Suggested change

	Assertions.assertThrows(NullPointerException.class, () -> {
	TypeName.BOOLEAN.annotated((AnnotationSpec[]) null);
	});
	assertThatThrownBy(() -> {
	TypeName.BOOLEAN.annotated((AnnotationSpec[]) null);
	})
	.isInstanceOf(NullPointerException.class);

[aldexis]

Indeed - this was updated by the excacvator and I didn't notice. I'll push a change