gitlab-ci: Add a make check step.

Invoke `make check` at the end of the default build, since we
don't do `make distcheck` for all the jobs. At the moment this
does nothing, but it should give better coverage if any test
harnesses are hooked up to the target.
5 jobs for master in 2 minutes and 31 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #10595
docker
autoconf

00:01:16

passed #10596
docker
fixed-point

00:00:49

passed #10599
docker
no-binaries

00:00:47

passed #10597
docker
no-float

00:00:51

passed #10598
docker
vorbis-psy

00:00:53