Integration tests against TP with multiple coinbase outputs

[Sjors]

After #57 it would be good to test against the recently added support for multiple coinbase outputs: stratum-mining/sv2-tp#55

It should be a round trip including submitting the solution, to make sure nothing goes wrong with serialisation.

[xyephy]

@GitGab19 you can assign me this since it related to #57 that I worked on.