tests: Add test for release_is_new()

Also refactor the function so we pass it the current version
and don't rely on the define inside of it.

This function only allows version format in the style of "x.y.z".

tests/unittests/unittests.c

@@ -635,6 +635,7 @@ main(int argc, char* argv[])
         cmocka_unit_test(prof_whole_occurrences_tests),
         cmocka_unit_test(prof_occurrences_of_large_message_tests),
         cmocka_unit_test(get_mentions_tests),
+        cmocka_unit_test(release_is_new_tests),
 
         cmocka_unit_test_setup_teardown(returns_no_commands,
                                         load_preferences,