gitlab-ci: Use xvfb-run

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.

.gitlab-ci/run_build.sh

@@ -11,12 +11,5 @@ cd build
 
 make V=1 VERBOSE=1
 
-Xvfb :99 &
+xvfb-run --auto-servernum make check V=1 VERBOSE=1
 
-set +e
-DISPLAY=:99 make check V=1 VERBOSE=1
-RESULT=$?
-set -e
-
-kill %Xvfb
-exit $RESULT

.gitlab-ci/run_distcheck.sh

@@ -9,17 +9,11 @@ mkdir build
 cd build
 ../configure
 
-Xvfb :99 &
-
-set +e
-DISPLAY=:99 make distcheck V=1 VERBOSE=1
-RESULT=$?
-set -e
+xvfb-run --auto-servernum make distcheck V=1 VERBOSE=1
 
 # If make distcheck failed don't even bother with the meson check, the tarball
 # may be invalid and it's just a waste
 if [ $RESULT -ne 0 ]; then
-    kill %Xvfb
     exit $RESULT
 fi
 
@@ -32,11 +26,6 @@ mkdir libglvnd
 tar -xf libglvnd-*.tar.gz -C libglvnd --strip-components 1
 pushd libglvnd
 meson builddir --auto-features=enabled
-set +e
-DISPLAY=:99 ninja -C builddir test
-RESULT=$?
-set -e
+xvfb-run --auto-servernum ninja -C builddir test
 popd
 
-kill %Xvfb
-exit $RESULT

.gitlab-ci/run_meson.sh

@@ -11,12 +11,4 @@ set -o xtrace
 meson build --unity=on --auto-features=enabled $CONFIGURE_OPTIONS
 ninja -C build
 
-Xvfb :99 &
-
-set +e
-DISPLAY=:99 ninja -C build test
-RESULT=$?
-set -e
-
-kill %Xvfb
-exit $RESULT
+xvfb-run --auto-servernum ninja -C build test