[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/errors/unresolvedReference.fir.kt.txt

@@ -1,16 +0,0 @@
-val test1: ErrorType
-  field = error("") /* ErrorCallExpression */
-  get
-
-val test2: ErrorType
-  field = error("") /* ErrorCallExpression */
-  get
-
-val test3: ErrorType
-  field = error("") /* ErrorCallExpression */56;
-  get
-
-val test4: ErrorType
-  field = error("") /* ErrorCallExpression */error("") /* ErrorExpression */;
-  get
-