[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/implicitNotNullOnDelegatedImplementation.kt.txt

@@ -1,5 +1,5 @@
 interface IFoo {
-  abstract fun foo(): String 
+  abstract fun foo(): String
 
 }
 
@@ -116,4 +116,3 @@ class TestK4 : IFoo {
   }
 
 }
-