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 gitlab-ci in 2 minutes and 30 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10590
docker
autoconf

00:01:18

passed #10591
docker
fixed-point

00:00:49

passed #10594
docker
no-binaries

00:00:47

passed #10592
docker
no-float

00:00:50

passed #10593
docker
vorbis-psy

00:00:53