test/common: unittest_fault_injector omits unit-main target

test/common: unittest_fault_injector omits unit-main target

unittest_fault_injector includes death tests for InjectAbort coverage,
and death tests don't play nicely with threads. the unit-main target
provides a main() function that initializes a g_ceph_context and starts
its associated threads. omit unit-main to prevent the log thread from
starting

Fixes: https://tracker.ceph.com/issues/71600
Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit bac78660c0338b33d092ab571f9347e6605aa0c4)