libnet: Initialize req_level in become_dc tests

libnet: Initialize req_level in become_dc tests

The net.api.become.dc tests would always pass the request into
libnet_vampire_cb_store_chunk() with req_level=0, which meant that
storing the chunk didn't use the correct replica_flags/exop.

I noticed this problem when working on client-side support for GET_TGT.
My changes relied on the critical-only request flag being passed down
into replmd, but because the request flags weren't passed correctly, my
changes caused the become_dc tests to fail.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>