sethostname tests: Fix link error on mingw.

sethostname tests: Fix link error on mingw.

* tests/test-sethostname1.c: New file, extracted from
tests/test-sethostname.c.
* tests/test-sethostname2.c: New file, extracted from
tests/test-sethostname.c.
* tests/test-sethostname.c: Remove file.
* modules/sethostname-tests (Files): Add tests/test-sethostname1.c,
tests/test-sethostname2.c. Remove tests/test-sethostname.c.
(Depends-on): Add gethostname.
(Makefile.am): Compile both test-sethostname1 and test-sethostname2.
Link the latter with $(GETHOSTNAME_LIB).