tls: add certificate list parsing tests

Add some comments to ca-roots.pem, to test that we handle comments
correctly in all places in a certificate list file, and add a new
ca-roots-bad.pem to verify that g_tls_certificate_list_new_from_file()
returns an error if given bad data.

https://bugzilla.gnome.org/show_bug.cgi?id=727692