tests: Add test for os interleaving
    
    We don't usually do tests purely for one change, but one change was
    special because when users will migrate to OVMF/AAVMF, commit 18f9f69b
    makes their lives easier by allowing them to interleave <type/> inside
    <os/>.  It would be nice of us to keep the possibility of them pasting
    the loader and nvram elements wherever it is valid, hence this test.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>