Add tests for market offers

Adding two tests for market offers. One creates selling offer, second creates buying offer. Both tests receive description from notary and check the data.

Please do  not merge, there is still problem with collision between those two tests.
We should not be making assertions about this value, if it is what I think it is.

This test should not be assuming that no other markets have been created on this notary until now.  When we make assumptions about state, it should be state we created ourselves.  In this case, it appears to be global and we can't control it without starting a new notary from scratch.  Which is probably not worth it just to verify this one thing.  Maybe just verify it's `>= 1` and be done with it?