Apply "final fields" Eclipse clean-up in spring-test

This has only been applied to `src/main/java`.

spring-test/src/main/java/org/springframework/mock/env/MockEnvironment.java

@@ -31,7 +31,7 @@ import org.springframework.core.env.ConfigurableEnvironment;
  */
 public class MockEnvironment extends AbstractEnvironment {
 
-	private MockPropertySource propertySource = new MockPropertySource();
+	private final MockPropertySource propertySource = new MockPropertySource();
 
 	/**
 	 * Create a new {@code MockEnvironment} with a single {@link MockPropertySource}.

spring-test/src/main/java/org/springframework/test/web/client/response/DefaultResponseCreator.java

@@ -40,7 +40,7 @@ import org.springframework.util.Assert;
  */
 public class DefaultResponseCreator implements ResponseCreator {
 
-	private HttpStatus statusCode;
+	private final HttpStatus statusCode;
 
 	private byte[] content = new byte[0];
 

spring-test/src/main/java/org/springframework/test/web/servlet/setup/AbstractMockMvcBuilder.java

@@ -56,7 +56,7 @@ import org.springframework.web.context.WebApplicationContext;
 public abstract class AbstractMockMvcBuilder<B extends AbstractMockMvcBuilder<B>>
 		extends MockMvcBuilderSupport implements ConfigurableMockMvcBuilder<B> {
 
-	private List<Filter> filters = new ArrayList<>();
+	private final List<Filter> filters = new ArrayList<>();
 
 	@Nullable
 	private RequestBuilder defaultRequestBuilder;

spring-test/src/main/java/org/springframework/test/web/servlet/setup/StandaloneMockMvcBuilder.java

@@ -137,7 +137,7 @@ public class StandaloneMockMvcBuilder extends AbstractMockMvcBuilder<StandaloneM
 	@Nullable
 	private Boolean removeSemicolonContent;
 
-	private Map<String, String> placeholderValues = new HashMap<>();
+	private final Map<String, String> placeholderValues = new HashMap<>();
 
 	private Supplier<RequestMappingHandlerMapping> handlerMappingFactory = RequestMappingHandlerMapping::new;