Try and reduce timing failures in desktop_browser new tab
The way this works at present, with a check_screen and then an
assert_and_click, there's a window where the check has passed
so we're committed to the assert, but it takes a half second or
so for the assert to actually complete (checking for a needle
is a somewhat heavy operation). During that half second the
'new update!' notification can...and quite often does...appear.
Changing the assert_and_click to a click_lastmatch should (I
hope) tighten this window; click_lastmatch should fire faster
than assert_and_click so there'll be less of a window for the
update notification to appear and break stuff.
Signed-off-by: Adam Williamson <awilliam@redhat.com>