test_chmod used to check that permission are 644, but this is not what is done
in the `process_resource` method. This function sets the permissions to the same
as the source file.

As a consequence, depending on your configuration (default umask for instance),
you may end up with the expected 644 permissions or… something else. (666 on our
gitlab-runner for instance).

To make the test consistent with what is done in the function, and consistent
with the initial purpose of this test [0], let's simply check that the
permissions of the source file and the compiled file are the same.

If it appears that this is not enough, a solution could be to enforce 644
permissions in the `process_resource` method, or give the expected permissions
through parameters.

[0]: https://www.cubicweb.org/ticket/17143773