Introduce FIR_IDENTICAL for FIR vs old frontend tests #KT-36879 Fixed

compiler/testData/diagnostics/tests/j+k/brokenCode/classDuplicates.fir.kt

@@ -1,13 +0,0 @@
-// FILE: Hello.kt
-private class Hello()
-{
-    val a = 4
-}
-
-fun test() {
-    // no exception is thrown (see KT-3897)
-    Hello().a
-}
-
-// FILE: Hello.java
-public class Hello {}