test/crimson: add delay when stopping seastar

test/crimson: add delay when stopping seastar

Seastar has bugs to have events undispatched during shutdown, which will
result in memory leak and thus fail LeakSanitizer.

Notably, there is a chance to lose the the destruction task of
foreign_ptr<lw_shared_ptr<conntrack::load_balancer>> submitted by
conntrack::~handle(). See
https://github.com/scylladb/seastar/blob/96ccbbe5262bed6d50a2392f6a77e84a490503b2/include/seastar/net/posix-stack.hh#L86-L89

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>