[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/enumWithSecondaryCtor.kt.txt

@@ -89,11 +89,10 @@ abstract enum class Test2 : Enum<Test2> {
     this/*Test2*/(x = 0)
   }
 
-  abstract fun foo() 
+  abstract fun foo()
 
   fun values(): Array<Test2> /* Synthetic body for ENUM_VALUES */
 
   fun valueOf(value: String): Test2 /* Synthetic body for ENUM_VALUEOF */
 
 }
-