tests: use --no-config

tests: use --no-config

Use --no-config to avoid loading arbitrary weston.ini files from unit
tests. It may affect the unit test results.

I actually hit the following case:

[13:34:04.636] Using config file '/home/pq/local/etc/weston.ini'
[13:34:04.636] Loading module '/home/pq/git/weston/.libs/headless-backend.so'
[13:34:04.637] launching '/home/pq/local/libexec/weston-keyboard'
[13:34:04.644] Loading module '/home/pq/local/lib/weston/desktop-shell.so'
[13:34:04.644] Loading module '/home/pq/local/lib/weston/xwayland.so'
[13:34:04.648] unlinking stale lock file /tmp/.X1-lock
[13:34:04.648] xserver listening on display :1
[13:34:04.648] Loading module '/home/pq/git/weston/.libs/./xwayland.so'
[13:34:04.648] xserver listening on display :2
[13:34:04.648] Module '/home/pq/local/lib/weston/xwayland.so' already loaded

Weston tries to load xwayland module three times, or which twice it
succeeds. This might not make the xwayland test end well. Or at all,
actually.

Adding --no-config should remove one of these loads of xwayland.so.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>