8039250: Test tools/javac/classfiles/InnerClasses/SyntheticClasses.java fails

Summary: The test should only analyze classfiles.
Reviewed-by: vromero

test/tools/javac/classfiles/InnerClasses/SyntheticClasses.java

@@ -41,7 +41,7 @@ public class SyntheticClasses {
 
     private void run() throws IOException, ConstantPoolException {
         File testClasses = new File(System.getProperty("test.classes"));
-        for (File classFile : testClasses.listFiles()) {
+        for (File classFile : testClasses.listFiles(f -> f.getName().endsWith(".class"))) {
             ClassFile cf = ClassFile.read(classFile);
             if (cf.getName().matches(".*\\$[0-9]+")) {
                 EnclosingMethod_attribute encl =