[FIR] Save IR dumps to .ir.txt files instead of .txt in tests

This is needed to avoid clashes between different dumps from different
  handlers

compiler/testData/ir/irText/expressions/funInterface/castFromAny.kt.txt

@@ -1,5 +1,5 @@
 fun interface KRunnable {
-  abstract fun invoke() 
+  abstract fun invoke()
 
 }
 
@@ -7,4 +7,3 @@ fun test(a: Any?) {
   a as Function0<Unit> /*~> Unit */
   a /*as Function0<Unit> */ /*-> KRunnable */.invoke()
 }
-