The x11 test is currently silently skipped in CI, because it requires a
running X server.
Create a xvfb wrapper to run the test. We do not use `xvfb-run`, because
it is a shell script and it causes valgrind to detect unrelated memory
issues in the shell (dash, bash).
Improve wrapper using a special ELF section
TODO: The wrapper is intended to be used with the x11comp test as well.