Added tests from trio's multierror test suite

[agronholm]

No description provided.

[njsmith]

Travis tests are failing due to a new pytest warning about unrecognized marks :-(

Could you undo the rename of _tests → tests? It makes the diff harder to read, and _tests is a better name anyway. (I know Trio still uses tests, but at some point we'll fix it: python-trio/trio#274)

Can you say a few words about how you adapted these from trio? Did they need many changes for the "v2" model?

[agronholm]

Could you undo the rename of _tests → tests? It makes the diff harder to read, and _tests is a better name anyway. (I know Trio still uses tests, but at some point we'll fix it: python-trio/trio#274)

The only reason I did this was because otherwise conftest.py will not get read by pytest. I assumed this was the reason why trio also had its tests directory named tests and not _tests.
Personally I am dead set against putting the tests inside the actual packages, and even more so against installing them along with the main code. But as they say: in Rome, do as romans do.

[agronholm]

The leftover problem that prevents the tests from passing on py3.6+ is the one about hashable exceptions. I will continue to investigate it and will report back here.

[agronholm]

Note to self: add fix from python-trio/trio#1023 to resolve the hashability issue.