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

compiler/testData/diagnostics/tests/syntheticExtensions/samAdapters/PassNull.fir.kt

@@ -1,10 +0,0 @@
-// FILE: KotlinFile.kt
-fun foo(javaInterface: JavaInterface) {
-    javaInterface.doIt(null, null) { }
-    javaInterface.doIt("", { }, null)
-}
-
-// FILE: JavaInterface.java
-public interface JavaInterface {
-    void doIt(String s, Runnable runnable1, Runnable runnable2);
-}

compiler/testData/diagnostics/tests/syntheticExtensions/samAdapters/PassNull.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // FILE: KotlinFile.kt
 fun foo(javaInterface: JavaInterface) {
     javaInterface.doIt(null, null) { }