Minor improvements in spring-test

- Remove final modifier from private method
- Use StandardCharsets
- Remove unnecessary toString calls
- Remove unnecessary static modifiers
- Refactor to use enhanced switch
- Replace concatenated strings with text blocks
- Rely on auto-boxing where appropriate
- Remove unnecessary code
- Fix imports in Kotlin test classes

Closes gh-29413

spring-test/src/main/java/org/springframework/test/context/junit/jupiter/AbstractExpressionEvaluatingCondition.java

@@ -93,7 +93,7 @@ abstract class AbstractExpressionEvaluatingCondition implements ExecutionConditi
 		AnnotatedElement element = context.getElement().get();
 		Optional<A> annotation = findMergedAnnotation(element, annotationType);
 
-		if (!annotation.isPresent()) {
+		if (annotation.isEmpty()) {
 			String reason = String.format("%s is enabled since @%s is not present", element,
 					annotationType.getSimpleName());
 			if (logger.isDebugEnabled()) {

spring-test/src/main/java/org/springframework/test/context/support/AbstractGenericContextLoader.java

@@ -188,7 +188,7 @@ public abstract class AbstractGenericContextLoader extends AbstractContextLoader
 	 * register a JVM shutdown hook for it
 	 * @return a new application context
 	 */
-	private final GenericApplicationContext loadContext(
+	private GenericApplicationContext loadContext(
 			MergedContextConfiguration mergedConfig, boolean forAotProcessing) throws Exception {
 
 		if (logger.isTraceEnabled()) {

spring-test/src/main/java/org/springframework/test/context/testng/AbstractTestNGSpringContextTests.java

@@ -201,7 +201,7 @@ public abstract class AbstractTestNGSpringContextTests implements IHookable, App
 	private Throwable getTestResultException(ITestResult testResult) {
 		Throwable testResultException = testResult.getThrowable();
 		if (testResultException instanceof InvocationTargetException) {
-			testResultException = ((InvocationTargetException) testResultException).getCause();
+			testResultException = testResultException.getCause();
 		}
 		return testResultException;
 	}

spring-test/src/main/java/org/springframework/test/util/XmlExpectationsHelper.java

@@ -82,7 +82,7 @@ public class XmlExpectationsHelper {
 	public void assertXmlEqual(String expected, String actual) throws Exception {
 		XmlUnitDiff diff = new XmlUnitDiff(expected, actual);
 		if (diff.hasDifferences()) {
-			AssertionErrors.fail("Body content " + diff.toString());
+			AssertionErrors.fail("Body content " + diff);
 		}
 	}