tests/acceptance: Tag NetBSD tests as 'os:netbsd'

tests/acceptance: Tag NetBSD tests as 'os:netbsd'

Avocado allows us to select set of tests using tags.
When wanting to run all tests using a NetBSD guest OS,
it is convenient to have them tagged, add the 'os:netbsd'
tag.

It allows one to run the NetBSD tests with:

 $ avocado --show=app,console run -t os:netbsd tests/acceptance/

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210623180021.898286-4-f4bug@amsat.org>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
[PMD: ammend the commit message with example command]
Signed-off-by: Cleber Rosa <crosa@redhat.com>