[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/classes/objectLiteralExpressions.kt.txt

@@ -1,5 +1,5 @@
 interface IFoo {
-  abstract fun foo() 
+  abstract fun foo()
 
 }
 
@@ -92,4 +92,3 @@ fun Outer.test4(): Inner {
     <no name provided>()
   }
 }
-