Parallelism in CI

[cwgoes]

I think we're not using parallism correctly; see https://circleci.com/docs/2.0/parallelism-faster-jobs/ - this would work if we split up tests into different containers, but at the moment it just means we run the tests multiple times (and slow down other builds).

See e.g. https://circleci.com/gh/cosmos/cosmos-sdk/10496#tests/containers/1, which just ran the same unit tests four different times.