tests: make $DISPLAY available for x11

I am not really sure why this passes in our CI ($DISPLAY wouldn't be set
there I wouldn't think?), but hopefully this fixes it.

Signed-off-by: Tycho Andersen <tycho@tycho.pizza>

test/backend/x11/conftest.py

@@ -184,6 +184,9 @@ def __init__(self, env, args=()):
         self.args = args
         self.core = Core
         self.manager = None
+        success, display = manager.c.eval("self.core.display_name")
+        assert success
+        self.env["DISPLAY"] = display
 
     def fake_click(self, x, y):
         """Click at the specified coordinates"""