gitlab CI: add meson to junit script

gitlab CI: add meson to junit script

This script was written by Emmanuele Bassi, copied from
https://gist.github.com/ebassi/e5296ec77ae9e0d3a33fd483b5613b09/

It converts meson test results into a junit file which we can then use to
display in the merge request GUI.

Note that as litest writes out junit files as well, some tests are reported
twice. Specifically: where litest fails the failure will be reported once
through litest itself and once by meson test. Oh well.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>