Bug 21281: (QA follow-up) Introduce _add_backtics

Bug 21281: (QA follow-up) Introduce _add_backtics

Added a (initially trivial) test in Creators/Lib.t too.
Sometimes you start something, but you end somewhere else ;)

The test seems a bit slower now (regex, lookahead, etc). But this should
not hurt label creators in daily use.
Advantages: code is even more solid, consolidated in one place and can be
tested on its own.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit db683b285cfb31deb9dbdafbc5a18e0b471ffa65)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 75b0e7a0787d82caa987afc67206fad2b5811e4c)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>