Fix remaining compiler warnings

Fix remaining Java compiler warnings, mainly around missing
generics or deprecated code.

Also add the `-Werror` compiler option to ensure that any future
warnings will fail the build.

Issue: SPR-11064

spring-context/src/test/java/org/springframework/aop/framework/CglibProxyTests.java

@@ -193,6 +193,11 @@ public final class CglibProxyTests extends AbstractAopProxyTests implements Seri
 				return MethodMatcher.TRUE;
 			}
 
+			@Override
+			public int hashCode() {
+				return 0;
+			}
+
 			@Override
 			public boolean equals(Object obj) {
 				return true;

spring-context/src/test/java/org/springframework/context/support/PropertySourcesPlaceholderConfigurerTests.java

@@ -177,6 +177,7 @@ public class PropertySourcesPlaceholderConfigurerTests {
 	}
 
 	@Test(expected=BeanDefinitionStoreException.class)
+	@SuppressWarnings("serial")
 	public void nestedUnresolvablePlaceholder() {
 		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
 		bf.registerBeanDefinition("testBean",
@@ -192,6 +193,7 @@ public class PropertySourcesPlaceholderConfigurerTests {
 	}
 
 	@Test
+	@SuppressWarnings("serial")
 	public void ignoredNestedUnresolvablePlaceholder() {
 		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
 		bf.registerBeanDefinition("testBean",

spring-context/src/test/java/org/springframework/scheduling/support/CronSequenceGeneratorTests.java

@@ -25,6 +25,7 @@ import static org.junit.Assert.*;
 /**
  * @author Juergen Hoeller
  */
+@SuppressWarnings("deprecation")
 public class CronSequenceGeneratorTests {
 
 	@Test