test/rgw: link lua test against unit-main for g_ceph_context

test/rgw: link lua test against unit-main for g_ceph_context

test_rgw_lua.cc was allocating a CephContext without initializing it, so
log statements from the tests/rgw code weren't actually being logged

the unit-main target links in the main() function from test/unit.cc
which initializes the CephContext via global_init(). use the resulting
global g_ceph_context instead of managing a separate one

Related-to: https://tracker.ceph.com/issues/70673
Signed-off-by: Casey Bodley <cbodley@redhat.com>