[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/firProblems/V8ArrayToList.kt.txt

@@ -12,4 +12,3 @@ fun box(): String {
   val list: List<String> = toList(array = array) as List<String>
   return list.get(index = 0)
 }
-