Set registerShutdownHook to false for tests

Update SpringApplicationContextLoader so that setRegisterShutdownHook
is false for tests.

Fixes gh-3763

Author: philwebb

spring-boot/src/main/java/org/springframework/boot/test/SpringApplicationContextLoader.java

@@ -82,6 +82,7 @@ public ApplicationContext loadContext(final MergedContextConfiguration config)
 			throws Exception {
 		assertValidAnnotations(config.getTestClass());
 		SpringApplication application = getSpringApplication();
+		application.setRegisterShutdownHook(false);
 		application.setMainApplicationClass(config.getTestClass());
 		application.setSources(getSources(config));
 		ConfigurableEnvironment environment = new StandardEnvironment();