[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/classMembers.kt.txt

@@ -61,7 +61,7 @@ class C {
   }
 
   interface NestedInterface {
-    abstract fun foo() 
+    abstract fun foo()
     fun bar() {
       return <this>.foo()
     }
@@ -78,4 +78,3 @@ class C {
   }
 
 }
-