This is treated by the test harness as though unset, i.e. we do normal
skipping and don't assume we are running in Meson's own project CI.
However, it has one distinction which is that it isn't an error to set
$CI without setting $MESON_CI_JOBNAME, if it is in fact set but to the
ignored value.
This lets automated workflows such as Linux distro testing, particularly
alpine linux, set $CI or have it set for them by default, without
messing things up.
Also it has the advantage of $CI actually enabling useful benefits! We
will still assume that this thirdparty environment wants to force
verbose logging (printing testlogs, running ninja/samu with -v) and
colorize the console.
a580fac83a