[java] New Rule: JUnit5TestNoPrivateModifier

[adangel]

Proposed Rule Name: JUnit5TestNoPrivateModifier

Proposed Category: Error Prone

Description:
See discussion in #3239. The rule should flag test methods / test classes, that are private, since they can't be executed by JUnit(5).

Sonar has a similar rule: https://rules.sonarsource.com/java/tag/tests/RSPEC-5810

The rule "JUnit5TestNoPrivateModifier" would probably also apply to JUnit4, so the name could also be "JUnitTestWrongModifier" (for JUnit4, private, protected and no modifier would be flagged, for JUnit5 only private modifier would be flagged).

Code Sample: This should include code, that should be flagged by the rule. If possible, the "correct" code
according to this new rule should also be demonstrated.

class MyTest { // not public, that's fine
    @Test
    private void testBad() { } // private test methods don't work
    @Test
    void testGoodJunit5() { } // package private as expected for JUnit5
    @Test
    public void testGoodJunit4() { } // public test method for JUnit4
}

Possible Properties:

• No properties. The rule should be smart enough to distinguish between JUnit5 and JUnit4 automatically.

[adangel]

@ajeans FYI - I've separated out the other rule into its own issue.