ERP5TypeFunctionalTestCase: set $XORG_LOCK_DIR

By setting XORG_LOCK_DIR, we can isolate processes when running on slapos test
nodes, so that instead of having all test runners used the shared /tmp, which
eventually fail with

    EnvironmentError: All displays locked : [':123', ':124', ':125', ...

errors when too many testnodes are currently running or too many testnodes have
left some stale lock files. Eventhough since fbdb4ec9 (test: Do not check
presence of lockfile in /tmp/ to assume Xvfb is running for this display, 2017-06-07)
things have improved, it happens that the lock files are owned by a different
unix user (because it was left by another testnode), then Xvfb is not able to
remove the stale lock file.

product/ERP5Type/tests/ERP5TypeFunctionalTestCase.py

@@ -145,6 +145,9 @@ class Xvfb(Process):
       """
 
   def run(self):
+    # set XORG_LOCK_DIR which is used by slapos pathed xorg, so that Xvfb and
+    # firefox don't use the system /tmp
+    os.environ['XORG_LOCK_DIR'] = os.environ['TMPDIR']
     for display_try in self.display_list:
       self._runCommand(display_try)
       if self.process.poll() is None: