The same problem as with the auth ones - the socket requestor needs to
be faked.

We need to fake the SocketRequestor in them. This also asked for little
update of the general base for socket requesting tests.

* It'll be needed in other tests as well, putting it to testutils.
* Needs documentation.
* A header only, to avoid linking order problems.

* The test fails
* It should at last return and get to some consistent state (eg. not
  listening on anything).

* They are requested and installed, instead of binding them locally
* Documentation needs update
* It'll fail in practice, since the requestor isn't initialized from
  auth and resolver.
* It needs to set the correct IPv6 flag

Only the test initialization and the singleton accessor functions are
implemented for now (and tests for those).

No need to create an opened socket and close it right away.

The dns_service is not tested, so the tests are not added. It is simple
anyway.

reviewed on jabber

details.  This should solve failure of some AXFR tests using SQLite3 on
some systems.

redundant 'pass'

to avoid duplicates.

counted twice.

section.  also extended tests to check the length the actual wire format
data for some specific cases.

account the DNS header size.  also added consideration for very large RRs
that would not even fit a single DNS message.

(I thought I did it in the IXFR tasks, but maybe I forgot to commit/push
that part)