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>

tests/desktop_browser.pm

@@ -9,7 +9,7 @@ sub _open_new_tab {
     # always visible because GNOME might pop up a notification that
     # blocks it. so, we try both.
     if (check_screen 'browser_new_tab') {
-        assert_and_click 'browser_new_tab';
+        click_lastmatch;
     }
     else {
         send_key 'ctrl-t';