Another attempt to improve robustness of desktop_browser

https://openqa.fedoraproject.org/tests/667693#step/desktop_browser/8
shows us matching on Save File when the window is in kind of a
borked state; we'd probably wind up clicking on Open with,
because by the time we click the content of the window will have
moved to where it's actually supposed to be...so let's try this
to slow it down a bit.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

tests/desktop_browser.pm

@@ -49,6 +49,7 @@ sub run {
     sleep 2;
     type_very_safely "https://kernel.org\n";
     assert_and_click "browser_kernelorg_patch";
+    wait_still_screen 2;
     assert_and_click "browser_download_save";
     sleep 2;
     send_key 'ret';