Add coverage stats to CI.

This patch adds a CI pipeline that builds the project with --coverage,
runs the unit tests and collects the coverage data with gcovr. A
coverage statistic is printed to the console log, which is extracted
with a regex. The current gitlab runners don't support SVE, so all
coverage statistics for files containing the string "sve" are excluded.

Unfortunately, gcovr's parser is unstable, so the docker image has been
fixed to a version that is known to work for us, in this case ubuntu
22.04.

Putting keywords in the the top section is deprecated, so all default
settings have been moved into a default section.

DEBIAN_FRONTENT=noninteractive is set, to prevent apt waiting for user
input.