test/crimson: do not use unit.cc as the driver of unittest_seastar_denc

test/crimson: do not use unit.cc as the driver of unittest_seastar_denc

as unit.cc initializes the CephContext and all of Ceph's infratructure,
which is not necessary for the denc test. also, seastar's builtin allocator
only pre-allocates 32 << 20 bytes. it's enough for the denc test, but
not necessarily enough for create CephContext and its friends. an option is
to use seastar's app template to initialize the memory allocator properly.
another option is to avoid initializing CephContext in this test.

the latter is simpler.

Signed-off-by: Kefu Chai <kchai@redhat.com>