Add a cold TestPublisher

[simonbasle]

Expected behavior

TestPublisher<String> result = TestPublisher.create();

assertThat(result.next("value").mono().block()).isEqualTo("value");

Actual behavior

Resolves to null, because by the time the block() is called, the DefaultTestPublisher is already terminated: it had no subscribers when calling next, so it only replays the terminal signal to subsequent subscribers.

Resolution

We need a "cold" implementation of TestPublisher that will replay a sequence to all its Subscribers. Possibly, let the user add signals (via next) or change the termination signal (via error() or complete()) between 2 subscriptions. In that case however, change should only impact the next new subscriber (ie. the publish methods don't impact current subscribers at all).

Reactor Core version

3.1.8+

[simonbasle]

cc @rwinch does it sound like what you need?