selftest/buildoptions: tag the download mirror test with 'yocto-mirrors'

selftest/buildoptions: tag the download mirror test with 'yocto-mirrors'

This will allow bundling all yocto mirror tests together, both for
the purposes of running only them specifically,
and excluding them from 'general' oe-selftest runs.

There is an upcoming test for sstate cache served over content
delivery network which will use the same tag, so it can be run
together with this.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>