libc/resolv: attempt to fix the test under WARNS=6

In a side-change that I'm working on to start defaulting src builds to
WARNS=6 where WARNS isn't otherwise specified, GCC6 (and clang, to a lesser
extent) pointed out a number of issues with the resolv tests:

• Global method variable that gets shadowed in run_tests()
• Signed/unsigned comparison between i in run_tests() and hosts->sl_cur

The shadowed variable looks like it might actually be bogus as written, as
we pass it to RUN_TESTS -> run_tests, but other parts use the global method
instead. This change is mainly geared towards correcting that by removing
the global and plumbing the method through from run_tests -> run into the
new thread.

For the signed/unsigned comparison, there's no compelling reason to not just
switch i/nthreads/nhosts to size_t.

The review also included a change to the load() function that was better
addressed by jhb in r365302.

Reviewed by: ngie, pstef
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24844