Use a different test object for funs

OTP24 apparently does not correctly recognize remote funs
(`fun mod:func/1`) in `is_fun`.

Author: filmor

rustler_tests/test/term_test.exs

@@ -77,7 +77,7 @@ defmodule RustlerTest.TermTest do
     assert RustlerTest.term_type([]) == :list
     assert RustlerTest.term_type({:ok, 42}) == :tuple
     assert RustlerTest.term_type(self()) == :pid
-    assert RustlerTest.term_type(&Function.identity/1) == :fun
+    assert RustlerTest.term_type(& &1) == :fun
     assert RustlerTest.term_type(make_ref()) == :reference
   end
 end