Add a test of bootchain stuff for updates
    
    This adds a new test intended to just check boot chain things
    for updates. It doesn't run any test modules besides the stock
    update ones, but sets a variable, ADVISORY_BOOT_TEST, which
    causes _advisory_update to do some additional stuff after
    installing the updates but before rebooting: it forces regen
    of the initramfs and bootloader config, and reinstalls the
    bootloader on BIOS (not UEFI as it's not relevant). If the
    following boot fails, we probably have a bug somewhere.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>