[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/callableReferences/boundInnerGenericConstructor.kt.txt

@@ -35,4 +35,3 @@ fun box(): String {
   val foo: Inner<String, String> = foo<String, String>(a = "O", b = "K", x = z::<init>/*<String>()*/)
   return foo.<get-a>().plus(other = foo.<get-b>())
 }
-