Move all the unit tests to /tests/<libname>/

Move all the unit tests to /tests/<libname>/

The unit tests are now using a common base, which means that
lib/dns/tests/ code now has to include lib/isc/include/isc/test.h and
link with lib/isc/test.c and lib/ns/tests has to include both libisc and
libdns parts.

Instead of cross-linking code between the directories, move the
/lib/<foo>/test.c to /tests/<foo>.c and /lib/<foo>/include/<foo>test.h
to /tests/include/tests/<foo>.h and create a single libtest.la
convenience library in /tests/.

At the same time, move the /lib/<foo>/tests/ to /tests/<foo>/ (but keep
it symlinked to the old location) and adjust paths accordingly.  In few
places, we are now using absolute paths instead of relative paths,
because the directory level has changed.  By moving the directories
under the /tests/ directory, the test-related code is kept in a single
place and we can avoid referencing files between libns->libdns->libisc
which is unhealthy because they live in a separate Makefile-space.

In the future, the /bin/tests/ should be merged to /tests/ and symlink
kept, and the /fuzz/ directory moved to /tests/fuzz/.

(cherry picked from commit 2c3b2dabe9a6b3c4a10f6498a1169f39ed031eed)