gitlab: Run tests with LP_NUM_THREADS=4 env var.

This forces mesa LLVMPIPE driver to use a fixed number of threads, and
greatly reduces its memory usage from the $(nproc) default. On Gitlab
this is 32 threads, which often exhaust VM space in 32bit tests.

tools/gitlab/test.yml

@@ -21,6 +21,7 @@
     - export PATH=$BASEDIR/usr/local/bin:$PATH
     - export DISPLAY=:0
     - export LC_ALL=C.UTF-8
+    - export LP_NUM_THREADS=4
     - export WINEDEBUG=err-all,fixme-all
     - |
       cat >$HOME/xorg.conf << EOF