tests/Makefile: add AVOCADO_TESTS option to make check-acceptance

Add the possibility of running all the tests from a single file, or
multiple files, running a single test within a file or multiple tests
within multiple files using `make check-acceptance` and the
AVOCADO_TESTS environment variable.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210923161141.232208-4-willianr@redhat.com>