Documentation/Notebooks on testing?

[matiboy]

Hi folks, just wondering if there is any documentation about writing tests.
Using the actual tests from the library I was able to figure out how to write my own tests but it feels like it could be useful to have a chapter on how to do that.
As an aside, can test observables be created using marbles instead of [on_next(...), ...]

Will gladly contribute on the testing docs if needed. Not sure if docs are being moved to the Notebooks?

[matiboy]

I went ahead and created the documentation in this PR

The sections are:

1. Basic example
2. Testing a custom operator
3. Timeline
4. An alternative using marbles
5. Testing an observable factory
6. Testing errors
7. Testing subscriptions, multiple observables, hot observables

All with (hopefully working) code samples.

Would love to hear feedback on the structure of the sections and the examples themselves