test-runner: make test_case_wait() wait for queued-work

Sometimes (for example with minimization) a request from the client
causes queued work rather than immediate work; so make the test client
'wait' command wait for a full frame cycle.

https://bugzilla.gnome.org/show_bug.cgi?id=737233
test-runner: represent the guard window as '|' for assert_stacking

Allow putting '|' into the list of windows for assert_stacking to
represent the position of the guard window. Not present is the same
as at the beginning (bottom) of the list.

https://bugzilla.gnome.org/show_bug.cgi?id=737233
tests: Add minimize/unminimize commands

Add commands to request the client to minimize or unminimize the window;
unminimize doesn't currently work for GTK+ because it expects XMapRequest
to be received by the window manager, but the window is already mapped.

https://bugzilla.gnome.org/show_bug.cgi?id=737233
Add a test for stacking vs. minimization

Test that the guard window is in the right place.

https://bugzilla.gnome.org/show_bug.cgi?id=737233
Fix stacking of the guard window

With the change to how hidden windows are stacked, the position
of the guard window with respect to the hidden windows got flipped
and the guard window was at the bottom of everything; fix it to
be on top of the hidden windows.

https://bugzilla.gnome.org/show_bug.cgi?id=737233