ClassSelectors for interfaces should be ignored

[marcphilipp]

When a ClassSelector for an interface type is passed as part of a discovery request, discovery currently fails with an exception:

Caused by: org.junit.platform.commons.JUnitException: TestEngine with ID 'testng' failed to discover tests
	at org.junit.platform.launcher.core.EngineDiscoveryOrchestrator.discoverEngineRoot(EngineDiscoveryOrchestrator.java:111)
	at org.junit.platform.launcher.core.EngineDiscoveryOrchestrator.discover(EngineDiscoveryOrchestrator.java:85)
	at org.junit.platform.launcher.core.DefaultLauncher.discover(DefaultLauncher.java:92)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:75)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.processAllTestClasses(JUnitPlatformTestClassProcessor.java:99)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.access$000(JUnitPlatformTestClassProcessor.java:79)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor.stop(JUnitPlatformTestClassProcessor.java:75)
	at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.stop(SuiteTestClassProcessor.java:61)
	... 25 more
Caused by: org.junit.platform.commons.JUnitException: ClassSelector [className = 'com.netflix.nebula.helloworld.HelloReplyOrBuilder'] resolution failed
	at org.junit.platform.launcher.listeners.discovery.AbortOnFailureLauncherDiscoveryListener.selectorProcessed(AbortOnFailureLauncherDiscoveryListener.java:39)
	at org.junit.platform.engine.support.discovery.EngineDiscoveryRequestResolution.resolveCompletely(EngineDiscoveryRequestResolution.java:102)
	at org.junit.platform.engine.support.discovery.EngineDiscoveryRequestResolution.run(EngineDiscoveryRequestResolution.java:82)
	at org.junit.platform.engine.support.discovery.EngineDiscoveryRequestResolver.resolve(EngineDiscoveryRequestResolver.java:113)
	at org.junit.support.testng.engine.TestNGTestEngine.discover(TestNGTestEngine.java:78)
	at org.junit.platform.launcher.core.EngineDiscoveryOrchestrator.discoverEngineRoot(EngineDiscoveryOrchestrator.java:103)
	... 32 more
Caused by: java.lang.NullPointerException
	at org.junit.support.testng.engine.TestAnnotationUtils.getClassHierarchy(TestAnnotationUtils.java:84)
	at org.junit.support.testng.engine.TestAnnotationUtils.collectTestAnnotations(TestAnnotationUtils.java:77)
	at org.junit.support.testng.engine.TestAnnotationUtils.collectGroups(TestAnnotationUtils.java:72)
	at org.junit.support.testng.engine.TestDescriptorFactory.createClassDescriptor(TestDescriptorFactory.java:38)
	at org.junit.support.testng.engine.TestNGSelectorResolver.lambda$resolve$0(TestNGSelectorResolver.java:45)

Instead, interfaces and abstract classes should be ignored.