Change the CI scripts to use xvfb-run instead of running Xvfb directly.
That avoids a potential race condition, where a test could fail because Xvfb
hasn't started listening for connections yet. xvfb-run will wait for Xvfb to be
ready before it runs any other programs.
That also simplifies error handling, because the CI scripts can just use
"set -e" instead of having to store the exit code, kill Xvfb, and then check
for an error.
Add a .gitlab-ci.yml file to run libglvnd's unit tests.
The "build-distcheck" target will simply run "make distcheck". That should also
cover running the unit tests for the x86-64 TLS build.
The "build-i386", "build-i386-tsd", and "build-x86-64-tsd" targets will build
and test the x86 TLS and TSD builds and the x86-64 TSD build.
The pure C stubs aren't covered yet, but they'll be easy to add once the unit
tests are fixed for them.
ARM and PPC builds also aren't covered yet.
