This saves time and bandwidth.

Closes jwilkes/purr-data#668

This causes tar_em_up.sh to download an older version of nw.js suitable
for OSX 10.8.

In my test, this reduced log size from 4.2MB to 3.2MB.

Example of the new output (taken from Gem compilation):

      CXX      libplugins_la-record.lo
    record.cpp:58:3: warning: #warning better handling of duplicate codecs [-Wcpp]
     # warning better handling of duplicate codecs
       ^~~~~~~
    record.cpp:87:3: warning: #warning isThreadable [-Wcpp]
     # warning isThreadable
       ^~~~~~~
    record.cpp:257:3: warning: #warning enumerateProperties [-Wcpp]
     # warning enumerateProperties
       ^~~~~~~
      CXX      libplugins_la-video.lo
      CXX      libplugins_la-videoBase.lo
      CXXLD    libplugins.la

Not all components honour the V=0 flag -- perhaps they don't use
Automake, or use a very old version. However, many do.

See also: https://www.gnu.org/software/automake/manual/html_node/Automake-Silent-Rules.html

See also: jwilkes/purr-data#666

The GitLab CI commands are now separated from the job definitions, using
YAML anchors.

The build and test commands should now show as separate, collapsible
sections in the job output (as documented at
https://docs.gitlab.com/ee/ci/pipelines/#custom-collapsible-sections).

The Debian image preparation is moved to before_script to separate it
from the build and test commands. Note that this adds an implicit 5 minute
time limit on this part of the script (see
https://gitlab.com/gitlab-org/gitlab-runner/-/issues/2716).

The commands are also now written once and copied using YAML anchors
(as documented at
https://docs.gitlab.com/ee/ci/yaml/README.html#yaml-anchors-for-before_script-and-after_script).

This is an initial attempt at a regression testing framework.

Initially it has some regression tests that come from the work on
improvements to the msg box.

The framework allows the tests themselves to remain extremely simple. They
just need to send a number and potential error message to the outlet.

However, they do need to be added in the main patch. This is because we
will need to run some tests in a certain order. We could number the test
filenames but that's less easy to change later.

matters. :(