[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/abstractMembers.kt.txt

@@ -5,7 +5,7 @@ abstract class AbstractClass {
 
   }
 
-  abstract fun abstractFun() 
+  abstract fun abstractFun()
   abstract val abstractVal: Int
     abstract get
 
@@ -16,7 +16,7 @@ abstract class AbstractClass {
 }
 
 interface Interface {
-  abstract fun abstractFun() 
+  abstract fun abstractFun()
   abstract val abstractVal: Int
     abstract get
 
@@ -25,4 +25,3 @@ interface Interface {
     abstract set
 
 }
-