tests: Add test for valid_tls_policy_option()

See last commit with comment regarding cons_show().

In this test here we can see that if we pass NULL cons_show() isn't
called. Making it a bit different that other invalid test cases.

tests/unittests/test_common.h

@@ -42,3 +42,4 @@ void test_strtoi_range_invalid_input(void** state);
 void test_strtoi_range_null_empty_input(void** state);
 void test_strtoi_range_null_err_msg(void** state);
 void test_string_matches_one_of_edge_cases(void** state);
+void test_valid_tls_policy_option(void** state);