[v3] port zarr-python 2 test suite to v3 branch

[jhamman]

We have largely been developing a parallel test suite in the v3 branch up until now. We recognize that there is a lot of good test coverage in the 2.0 test suite and we'd like to bring in as much as possible. This may be a big lift but its an important one.

@d-v-b - you've been thinking about this already, can you lay out a plan here?