Causality testing API

[teivah]

The quality of the tests must be improved. In my opinion, we must get rid of any single time.Sleep which leads to non-deterministic tests.

As a first proposition, I created this PR: #206

Basically, we can use a DSL this way:

observables := mockObservables(t, `
1
	2
3
	4
x
	x
`)

Each column represents an observable emitting item. The first observable will produce the items 1, 3 and will close (x) whereas the second observable will produce the items 2, 4 and will close.

This code will produce observables from which iterating over the internal iterator where causality is guaranteed. It means 1 will be handled before 2, 2 before 3 etc. regardless of the execution context. It is also possible to send an error item with the character e.

[teivah]

Dropped from the v2