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.

Closes #2801 (closed)