[java decompiler] improves anonymous classes verification

- puts the check under an option
- uses 'EnclosingMethod' attribute to skip unrelated methods

test/org/jetbrains/java/decompiler/SingleClassesTest.java

@@ -24,7 +24,8 @@ public class SingleClassesTest {
     fixture = new DecompilerTestFixture();
     fixture.setUp(IFernflowerPreferences.BYTECODE_SOURCE_MAPPING, "1",
                   IFernflowerPreferences.DUMP_ORIGINAL_LINES, "1",
-                  IFernflowerPreferences.IGNORE_INVALID_BYTECODE, "1");
+                  IFernflowerPreferences.IGNORE_INVALID_BYTECODE, "1",
+                  IFernflowerPreferences.VERIFY_ANONYMOUS_CLASSES, "1");
   }
 
   @After