Avoid String concatenation for lookup in StaticMessageSource

Closes gh-22451

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

@@ -59,7 +59,6 @@ public class StaticMessageSourceTests extends AbstractApplicationContextTests {
 	@Test
 	@Override
 	public void count() {
-		// These are only checked for current Ctx (not parent ctx)
 		assertCount(15);
 	}