Provide test instance to TestTemplate extensions

Issue: #419

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/TestTemplateContainerExtensionContext.java

@@ -28,9 +28,13 @@
 final class TestTemplateContainerExtensionContext extends AbstractExtensionContext<TestTemplateTestDescriptor>
 		implements ContainerExtensionContext {
 
+	private final Object testInstance;
+
 	TestTemplateContainerExtensionContext(ExtensionContext parent, EngineExecutionListener engineExecutionListener,
-			TestTemplateTestDescriptor testDescriptor) {
+			TestTemplateTestDescriptor testDescriptor, Object testInstance) {
+
 		super(parent, engineExecutionListener, testDescriptor);
+		this.testInstance = testInstance;
 	}
 
 	@Override
@@ -45,8 +49,7 @@ public Optional<Class<?>> getTestClass() {
 
 	@Override
 	public Optional<Object> getTestInstance() {
-		// TODO Determine how to access the test instance
-		return null;
+		return Optional.ofNullable(this.testInstance);
 	}
 
 	@Override

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/TestTemplateTestDescriptor.java

@@ -57,13 +57,17 @@ public boolean mayRegisterTests() {
 	public JupiterEngineExecutionContext prepare(JupiterEngineExecutionContext context) throws Exception {
 		ExtensionRegistry registry = populateNewExtensionRegistryFromExtendWith(getTestMethod(),
 			context.getExtensionRegistry());
-		ContainerExtensionContext testExtensionContext = new TestTemplateContainerExtensionContext(
-			context.getExtensionContext(), context.getExecutionListener(), this);
+
+		// The test instance should be properly maintained by the enclosing class's ExtensionContext.
+		Object testInstance = context.getExtensionContext().getTestInstance().orElse(null);
+
+		ContainerExtensionContext containerExtensionContext = new TestTemplateContainerExtensionContext(
+			context.getExtensionContext(), context.getExecutionListener(), this, testInstance);
 
 		// @formatter:off
 		return context.extend()
 				.withExtensionRegistry(registry)
-				.withExtensionContext(testExtensionContext)
+				.withExtensionContext(containerExtensionContext)
 				.build();
 		// @formatter:on
 	}