Position e2e browser at top left corner of screen

When taking screenshots of the test process running under xvfb it's
offset from the top left corner, the default geometry of qutebrowser is
800x600+50+50. The default size of pytest-xvfb is 800x600, which means
part of the browser window is outside the X11 screen and doesn't get
captured.

This commit duplicates the width and height from the default geometry in
mainwindow.py but sets the x and y offsets to zero so that the browser
window is fully contained within the X11 window.

tests/end2end/fixtures/quteprocess.py

@@ -415,7 +415,8 @@ class QuteProc(testprocess.Process):
                 '--debug-flag', 'werror',
                 '--debug-flag', 'test-notification-service',
                 '--debug-flag', 'caret',
-                '--qt-flag', 'disable-features=PaintHoldingCrossOrigin']
+                '--qt-flag', 'disable-features=PaintHoldingCrossOrigin',
+                '--qt-arg', 'geometry', '800x600+0+0']
 
         if self.request.config.webengine and testutils.disable_seccomp_bpf_sandbox():
             args += testutils.DISABLE_SECCOMP_BPF_ARGS