MI: reinstate init_ep and crc32 functions for use in test

MI: reinstate init_ep and crc32 functions for use in test

Ideally, we would be using the actual implementation of
nvme_mi_init_ep() and nvme_mi_crc32_update for our tests, rather than
open-coding it in the test init.

This change exports nvme_mi_init_ep and nvme_mi_crc32_update from
libnvme-mi.so, but both remain excluded from the headers, as they are
only intended for use in the transport API. This means we can call them
from the tests, but keep somewhat-internal.

We put this into a specific _TEST section of the version script, to keep
separate from the public symbols, and add a comment to suit.

This prevents us from diverging the endpoint init process in our
testcases.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>