Fix failures in isc netmgr_test on big endian machines

Fix failures in isc netmgr_test on big endian machines

Typing from libuv structure to isc_region_t is not possible, because
their sizes differ on 64 bit architectures. Little endian machines seems
to be lucky and still result in test passed. But big endian machine such
as s390x fails the test reliably.

Fix by directly creating the buffer as isc_region_t and skipping the
type conversion. More readable and still more correct.

(cherry picked from commit 057438cb45f2f02615dc309e3822f23c0ca70a0a)