Asynchronous test cases can be delared using SEASTAR_TEST_CASE macro,
which is equivalent to BOOST_AUTO_TEST_CASE, but the function body is
run inside a reactor and can returns a future<> which resolves when
the test is done.
It allows for simpler client code because the client can assume that
when cancel() returns the callback will not run and thus there's no
need to handle the race between timer teardown and execution of the
callback.
The imperative form suggests that in addition to returning a future it
performs some action and thus is needed regardless of whether we want
to add a callback or not. But in fact it does not do anything, just
gives away a future.
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
