Use xfvb-run for javascript tests

There's no need for the complicated Xvfb setup/teardown commands when
running the Javascript test suite, when the `xvfb-run` command does
all this for us.

tox.ini

@@ -93,12 +93,10 @@ commands =
 [testenv:javascript]
 usedevelop=True
 deps = libsass==0.15.1
+allowlist_externals = xvfb-run
 commands_pre =
 commands =
-         /sbin/start-stop-daemon -o --stop --quiet --pidfile /var/tmp/xvfb.pid
-         /sbin/start-stop-daemon --start --quiet --pidfile /var/tmp/xvfb.pid --make-pidfile --background --exec /usr/bin/Xvfb -- {env:DISPLAY} -screen 0 1024x768x24 -fbdir /var/tmp -ac
-         {toxinidir}/tests/javascript-test.sh "{toxinidir}"
-         /sbin/start-stop-daemon --stop --quiet --pidfile /var/tmp/xvfb.pid
+         xvfb-run {toxinidir}/tests/javascript-test.sh "{toxinidir}"
 
 [testenv:pylint]
 deps = pip-tools