Use the new ability to report full mappings after the discovery and
allow using `test_reference_resolver_class` to specify the resolver
class intended for test discovery.

To allow sufficient flexibility also support
`test_reference_resolver_args` to override resolver arguments and
`test_reference_resolver_extra` to extend the resolver extra_params.

Note the overridden arguments apply only to the resolver, not to whole
Avocado so it's not possible to override wrappers or such just for a
single test execution. Anyway it is possible to set `vt_config` just for
that single loader, which is more-than useful.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>