[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/stubs/kotlinInnerClass.ir.txt

@@ -0,0 +1,7 @@
+FILE fqName:<root> fileName:/kotlinInnerClass.kt
+  FUN name:test visibility:public modality:FINAL <> (inner:<root>.Outer.Inner) returnType:kotlin.Unit
+    VALUE_PARAMETER name:inner index:0 type:<root>.Outer.Inner
+    BLOCK_BODY
+      RETURN type=kotlin.Nothing from='public final fun test (inner: <root>.Outer.Inner): kotlin.Unit declared in <root>'
+        CALL 'public final fun foo (): kotlin.Unit declared in <root>.Outer.Inner' type=kotlin.Unit origin=null
+          $this: GET_VAR 'inner: <root>.Outer.Inner declared in <root>.test' type=<root>.Outer.Inner origin=null