Fail fast if @WebAppConfiguration is used with a non-mock web environement

@WebAppConfiguration expects a mock web environment. If it is used
in conjuction with @SpringBootTest configured with a RANDOM_PORT or
DEFINED_PORT web environment a null pointer exception occurs as an
assumption that's made by MockServerContainerContextCustomizer doesn't
hold true in a non-mock web environment.

This commit updates SpringBootTestContextBootstrap to detect the
illegal configuration combination and fail fast, advising the user
to remove @WebAppConfiguration or reconfigure @SpringBootTest.

Closes gh-6795

spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootTestContextBootstrapper.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.test.context;
 
+import java.lang.annotation.Annotation;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -75,6 +76,7 @@ public class SpringBootTestContextBootstrapper extends DefaultTestContextBootstr
 	@Override
 	public TestContext buildTestContext() {
 		TestContext context = super.buildTestContext();
+		verifyConfiguration(context.getTestClass());
 		WebEnvironment webEnvironment = getWebEnvironment(context.getTestClass());
 		if (webEnvironment == WebEnvironment.MOCK && hasWebEnvironmentClasses()) {
 			context.setAttribute(ACTIVATE_SERVLET_LISTENER, true);
@@ -261,6 +263,24 @@ public class SpringBootTestContextBootstrapper extends DefaultTestContextBootstr
 		return AnnotatedElementUtils.getMergedAnnotation(testClass, SpringBootTest.class);
 	}
 
+	protected void verifyConfiguration(Class<?> testClass) {
+		SpringBootTest springBootTest = getAnnotation(testClass);
+		if (springBootTest != null
+				&& (springBootTest.webEnvironment() == WebEnvironment.DEFINED_PORT
+						|| springBootTest.webEnvironment() == WebEnvironment.RANDOM_PORT)
+				&& getAnnotation(WebAppConfiguration.class, testClass) != null) {
+			throw new IllegalStateException("@WebAppConfiguration should only be used "
+					+ "with @SpringBootTest when @SpringBootTest is configured with a "
+					+ "mock web environment. Please remove @WebAppConfiguration or "
+					+ "reconfigure @SpringBootTest.");
+		}
+	}
+
+	private <T extends Annotation> T getAnnotation(Class<T> annotationType,
+			Class<?> testClass) {
+		return AnnotatedElementUtils.getMergedAnnotation(testClass, annotationType);
+	}
+
 	/**
 	 * Create a new {@link MergedContextConfiguration} with different classes.
 	 * @param mergedConfig the source config