Tweak needle matches at start of _do_install_and_reboot

The way this was set up before, if `anaconda_main_hub` matched
immediately but some spoke was still in a 'processing' state,
it only had 30 seconds (default `assert_and_click` timeout) to
complete and allow the 'Begin Installation' button to appear.
It seems unnecessary to match on *both* needles, really, so
let's just give 300 seconds for the `begin_installation` needle
to appear. It's not going to appear on any other screen.

This problem caused a couple of spurious failures today -
https://openqa.fedoraproject.org/tests/77839 and
https://openqa.fedoraproject.org/tests/77858 - because they
took a bit too long for the INSTALLATION DESTINATION spoke to
clear.

tests/_do_install_and_reboot.pm

@@ -5,14 +5,12 @@ use utils;
 
 sub run {
     my $self = shift;
-    # Anaconda hub
-    assert_screen "anaconda_main_hub", 300; #
-
     # Begin installation
     # Sometimes, the 'slide in from the top' animation messes with
     # this - by the time we click the button isn't where it was any
     # more. So wait a sec just in case.
-    sleep 1;
+    assert_screen "anaconda_main_hub_begin_installation", 300; #
+    wait_still_screen 2;
     assert_and_click "anaconda_main_hub_begin_installation";
 
     # Set root password