Makes junit:junit optional.

This means modules using spring-cloud-test-support must have the
junit-vintage-engine dependency so tests will run properly.

spring-cloud-context-integration-tests/pom.xml

@@ -60,6 +60,11 @@
 			<artifactId>spring-boot-configuration-processor</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.junit.vintage</groupId>
+			<artifactId>junit-vintage-engine</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 	<build>
 		<plugins>

spring-cloud-context-webflux-integration-tests/pom.xml

@@ -42,6 +42,11 @@
 			<artifactId>spring-boot-configuration-processor</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.junit.vintage</groupId>
+			<artifactId>junit-vintage-engine</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 	<build>
 		<plugins>

spring-cloud-loadbalancer/pom.xml

@@ -117,5 +117,10 @@
 			<artifactId>awaitility</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.junit.vintage</groupId>
+			<artifactId>junit-vintage-engine</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 </project>

spring-cloud-loadbalancer/src/test/java/org/springframework/cloud/loadbalancer/security/OAuth2LoadBalancerClientAutoConfigurationTests.java

@@ -18,7 +18,9 @@ package org.springframework.cloud.loadbalancer.security;
 
 import java.net.URI;
 
+import org.apache.catalina.webresources.TomcatURLStreamHandlerFactory;
 import org.junit.After;
+import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
@@ -51,6 +53,14 @@ public class OAuth2LoadBalancerClientAutoConfigurationTests {
 	@Rule
 	public ExpectedException expected = ExpectedException.none();
 
+	@Before
+	public void before() {
+		// FIXME: why do I need to do this? (fails in maven build without it.
+		// https://stackoverflow.com/questions/28911560/tomcat-8-embedded-error-org-apache-catalina-core-containerbase-a-child-con
+		// https://github.com/spring-projects/spring-boot/issues/21535
+		TomcatURLStreamHandlerFactory.disable();
+	}
+
 	@After
 	public void close() {
 		if (this.context != null) {

spring-cloud-test-support/pom.xml

@@ -54,6 +54,7 @@
 		<dependency>
 			<groupId>junit</groupId>
 			<artifactId>junit</artifactId>
+			<optional>true</optional>
 		</dependency>
 		<dependency>
 			<groupId>org.springframework</groupId>