Tests: change ctdb_takeover_tests.c to include ctdbd code

Tests: change ctdb_takeover_tests.c to include ctdbd code

Do this instead of linking to it.  This means that, after previous
cleanups, we can fix ctdb_takeover.c to use static functions when
appropriate and simply include all the code we need to run tests.

To make this reusable in other tests, new file ctdbd_tests.c does all
of the relevant including.  ctdb_takeover_tests.c just includes that
file.

Test objects built in this way can depend on new Makefile macro
$(CTDB_TEST_C), which contains ctdbd_tests.c and everything it
includes.

Signed-off-by: Martin Schwenke <martin@meltin.net>
(This used to be ctdb commit 41869d42194b74db43a176a068e96e411007e5f2)