test/crimson/seastore: teardown in reactor

test/crimson/seastore: teardown in reactor

otherwise, we rely on the destructor of TMTestState to teardown the
fixuture created in TMTestState::_init(), but TMTestState::_init() is
called in reactor. the objects like seastar::metric_groups are
supposed to be destroyed on the same thread where they are created.
because they use thread local storage of storing persisting their status.
if we destroy objects like seastar::metric_groups on different reactor
or thread where they are created, we would have memory leak and
unexpected behavior.

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