Use file based loader with MockServletContext

Update SpringApplicationContextLoader so that the MockServletContext
uses FileSystemResourceLoader. This allows `/src/main/webapp` folder
to be found.

See spring-projectsgh-2654

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

@@ -39,6 +39,7 @@
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.env.MapPropertySource;
 import org.springframework.core.env.StandardEnvironment;
+import org.springframework.core.io.FileSystemResourceLoader;
 import org.springframework.mock.web.MockServletContext;
 import org.springframework.test.context.ContextConfigurationAttributes;
 import org.springframework.test.context.ContextLoader;
@@ -261,7 +262,8 @@ private void addMockServletContext(
 				List<ApplicationContextInitializer<?>> initializers,
 				WebMergedContextConfiguration webConfiguration) {
 			MockServletContext servletContext = new MockServletContext(
-					webConfiguration.getResourceBasePath());
+					webConfiguration.getResourceBasePath(),
+					new FileSystemResourceLoader());
 			initializers.add(0, new ServletContextApplicationContextInitializer(
 					servletContext));
 		}