[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/expressions/arrayAugmentedAssignment2.kt.txt

@@ -1,10 +1,10 @@
 interface IA {
-  abstract operator fun get(index: String): Int 
+  abstract operator fun get(index: String): Int
 
 }
 
 interface IB {
-  abstract operator fun IA.set(index: String, value: Int) 
+  abstract operator fun IA.set(index: String, value: Int)
 
 }
 
@@ -15,4 +15,3 @@ fun IB.test(a: IA) {
     (<this>, tmp0_array).set(index = tmp1_index0, value = tmp0_array.get(index = tmp1_index0).plus(other = 42))
   }
 }
-