Fix regressions in two unit tests, because of a CSS addition from here:
https://we.phorge.it/rP349f006904fabf1d4df31ff4840502af3ab379a7
https://we.phorge.it/D25118
Some unit tests were missing the new CSS class 'remarkup-link-ext'. Added.
Also, we do not consider anymore 'mailto:info@example.com' as an internal link.
We hardened a bit the unit tests to also include mobile numbers in links, and alien protocols.
Closes T15967
Closes T15973
Closes T15974