[gnome-settings-daemon] tests: Fix mutter >= 3.26 not starting up

[Bastien Nocera <hadess src gnome org>]

commit 8351ff4d451da2004efc36a205e369b3207ecfe6
Author: Bastien Nocera <hadess hadess net>
Date:   Mon Feb 5 16:34:56 2018 +0100

    tests: Fix mutter >= 3.26 not starting up
    
    If mutter is built with Wayland support, it will fail to start up under
    our mocked logind because it cannot get information about the session:
    mutter-WARNING **: Can't initialize KMS backend: Could not get session ID: No such file or directory
    
    See https://gitlab.gnome.org/GNOME/mutter/merge_requests/15
    
    https://bugzilla.gnome.org/show_bug.cgi?id=793189

 tests/gsdtestcase.py |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/tests/gsdtestcase.py b/tests/gsdtestcase.py
index 81691b5..31f65e9 100644
--- a/tests/gsdtestcase.py
+++ b/tests/gsdtestcase.py
@@ -170,7 +170,9 @@ class GSDTestCase(dbusmock.DBusTestCase):
         ''' start mutter '''
 
         klass.mutter_log = open(os.path.join(klass.workdir, 'mutter.log'), 'wb')
-        klass.mutter = subprocess.Popen(['mutter'],
+        # See https://gitlab.gnome.org/GNOME/mutter/merge_requests/15
+        os.environ['XDG_SESSION_TYPE'] = 'x11'
+        klass.mutter = subprocess.Popen(['mutter', '--x11'],
                                          stdout=klass.monitor_log,
                                          stderr=subprocess.STDOUT)