[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/superCallsComposed.kt.txt

@@ -15,7 +15,7 @@ open class Base {
 }
 
 interface BaseI {
-  abstract fun foo() 
+  abstract fun foo()
   abstract val bar: String
     abstract get
 
@@ -38,4 +38,3 @@ class Derived : Base, BaseI {
     }
 
 }
-