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

compiler/testData/diagnostics/tests/cast/neverSucceeds/CastToNotNullSuper.fir.kt

@@ -1,8 +0,0 @@
-open class A {
-    fun foo() {}
-}
-class B : A()
-
-fun test(b: B?) {
-    (b as A).foo()
-}

compiler/testData/diagnostics/tests/cast/neverSucceeds/CastToNotNullSuper.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 open class A {
     fun foo() {}
 }