Avoid reporting discovery warnings for abstract inner classes with tests

While aee7c88 fixed the issue when
nested classes were discovered via `NestedClassSelector` or their parent
class, passing a `ClassSelector` for them still caused a warning.

Fixes #4635.

(cherry picked from commit 4305ad8)

Author: marcphilipp

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/predicates/TestClassPredicates.java

@@ -63,7 +63,7 @@ public TestClassPredicates(DiscoveryIssueReporter issueReporter) {
 				.and(isInner(issueReporter));
 		this.isValidStandaloneTestClass = isNotPrivateUnlessAbstract("Test", issueReporter) //
 				.and(isNotLocal(issueReporter)) //
-				.and(isNotInner(issueReporter)) // or should be annotated with @Nested!
+				.and(isNotInnerUnlessAbstract(issueReporter)) //
 				.and(isNotAnonymous(issueReporter));
 	}
 
@@ -127,8 +127,8 @@ private static Condition<Class<?>> isInner(DiscoveryIssueReporter issueReporter)
 		});
 	}
 
-	private static Condition<Class<?>> isNotInner(DiscoveryIssueReporter issueReporter) {
-		return issueReporter.createReportingCondition(testClass -> !isInnerClass(testClass),
+	private static Condition<Class<?>> isNotInnerUnlessAbstract(DiscoveryIssueReporter issueReporter) {
+		return issueReporter.createReportingCondition(testClass -> !isInnerClass(testClass) || isAbstract(testClass),
 			testClass -> createIssue("Test", testClass, "must not be an inner class unless annotated with @Nested"));
 	}
 

jupiter-tests/src/test/java/org/junit/jupiter/engine/NestedTestClassesTests.java

@@ -231,12 +231,20 @@ void individualMethodsWithinRecursiveNestedTestClassHierarchiesAreExecuted() {
 	}
 
 	@Test
-	void doesNotReportDiscoveryIssueForAbstractInnerClass() {
+	void doesNotReportDiscoveryIssueForClassWithAbstractInnerClass() {
 		var discoveryIssues = discoverTestsForClass(ConcreteWithExtendedInnerClassTestCase.class).getDiscoveryIssues();
 
 		assertThat(discoveryIssues).isEmpty();
 	}
 
+	@Test
+	void doesNotReportDiscoveryIssueForAbstractInnerClass() {
+		var discoveryIssues = discoverTestsForClass(
+			AbstractBaseWithInnerClassTestCase.AbstractInnerClass.class).getDiscoveryIssues();
+
+		assertThat(discoveryIssues).isEmpty();
+	}
+
 	private void assertNestedCycle(Class<?> start, Class<?> from, Class<?> to) {
 		var results = executeTestsForClass(start);
 		var expectedMessage = String.format(