tests: Add more tests for firmware selection

tests: Add more tests for firmware selection

Most of these are just additional coverage, but a few demonstrate
bugs in libvirt:

  * firmware-manual-efi-nvram-template-nonstandard sees the NVRAM
    template path, which was explicitly provided in the XML,
    being overridden by the firmware selection machinery;

  * firmware-auto-efi-rw* and firmware-manual-efi-rw-legacy-paths
    lose the loader.readonly=no setting and thus behave
    differently than requested;

  * firmware-manual-efi-loader-path-nonstandard fails because an
    NVRAM path doesn't get generated.

We're going to address all these issues with upcoming changes.

Note that the firmware-auto-efi-nvram-template-nonstandard
failure is expected: firmware autoselection has been enabled, but
the NVRAM template points to a custom path that's not mentioned
in any of the firmware descriptors and so it can't succeed.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>