consolidate login waits, use postinstall not entrypoint for base
    
    Summary:
    I started out wanting to fix an issue I noticed today where
    graphical upgrade tests were failing because they didn't wait
    for the graphical login screen properly; the test was sitting
    at the 'full Fedora logo' state of plymouth for a long time,
    so the current boot_to_login_screen's wait_still_screen was
    triggered by it and the function wound up failing on the
    assert_screen, because it was still some time before the real
    login screen appeared.
    
    So I tweaked the boot_to_login_screen implementation to work
    slightly differently (look for a login screen match, *then* -
    if we're dealing with a graphical login - wait_still_screen
    to defeat the 'old GPU buffer showing login screen' problem
    and assert the login screen again). But while working on it,
    I figured we really should consolidate all the various places
    that handle the bootloader -> login, we were doing it quite
    differently in all sorts of different places. And as part of
    that, I converted the base tests to use POSTINSTALL (and thus
    go through the shared _wait_login tests) instead of handling
    boot themselves. As part of *that*, I tweaked main.pm to not
    require all POSTINSTALL tests have the _postinstall suffix on
    their names, as it really doesn't make sense, and renamed the
    tests.
    
    Test Plan: Run all tests, see if they work.
    
    Reviewers: jskladan, garretraziel
    
    Reviewed By: garretraziel
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D1015