They need one go package which I can't find even in Debian,
so it probably can't work without network access.
The new dnstap in extra_tests runs if dnstap is built and go is found.
It also tries to keep the source tree clean.

Now both query and reply messages are tested.

In CI (after caching go deps in image) this tests only takes slightly
more time than the longest config.* tests, so that seems OK.
Even so, it's not added into the valgrind variant, as compilation
of the test still isn't split away from the run itself.