This adds a test for QA:Testcase_Anaconda_User_Interface_VNC -
the VNC install test case. It's implemented as a server/client
pair, with the server booting from the Server DVD image with
`inst.vnc` and the client booting from the desktop base disk
image, setting up networking, then running Boxes to connect to
the server and run the install.
There are various little tweaks to test loading and logic to
handle this, mostly pretty clear. We also move the workaround
for 'spurious auth prompt appears on desktop after you switch
away to a VT and back' out of the desktop update test and into
the `desktop_vt` helper function, since now this test can hit
it as well. We enhance _graphical_wait_login to handle the boot
loader if needed (it has never needed to until now).
Signed-off-by: Adam Williamson <awilliam@redhat.com>
