Fix EnvironmentError: All displays locked : [':123', ':124', ':125', .. on test node

@luke @rafael @vincentB see commit message:

test: Do not check presence of lockfile in /tmp/ to assume Xvfb is running for this display

This caused two problems:

• If Xfvb was killed and left a stale lock file, we don't reuse this display. After some time we end up having "All display locked" exception but no Xvfb process running.
• Xvfb's slapos is patched to store control sockets and lockfiles in $TMPDIR ( https://lab.nexedi.com/nexedi/slapos/commit/58462c2415b3a71c3f9ee7fdc290cf09ddb0adc0 ). We don't have $TMPDIR set in testnodes, but this would be a problem if we ever set it.

Because Xvfb can detect a stale lock file upon startup and start anyway, the easy way running Xvfb for each display and see if it's still running after a few seconds.