Add assertions on utility classes in IP module

* Add assertions and comment on the utility class.
Added assertion on the utility class, so that it can't be mistakenly ever initialized in the class.
* Add exception message for RegexUtils
* Add exception message "Class Instantiation not allowed." for the TestingUtilities

spring-integration-ip/src/main/java/org/springframework/integration/ip/util/RegexUtils.java

@@ -43,7 +43,9 @@ public final class RegexUtils {
 		return stringToEscape.replaceAll("([.$\\[\\]^*+{}()\\\\?|])", "\\\\$1");
 	}
 
+	// Non-instantiable utility class
 	private RegexUtils() {
+		throw new AssertionError("Class Instantiation not allowed.");
 	}
 
 }

spring-integration-ip/src/main/java/org/springframework/integration/ip/util/TestingUtilities.java

@@ -33,7 +33,9 @@ import org.springframework.lang.Nullable;
  */
 public final class TestingUtilities {
 
+	// Non-instantiable utility class
 	private TestingUtilities() {
+		throw new AssertionError("Class Instantiation not allowed.");
 	}
 
 	/**