Separate Wolfram Language build and test jobs on CI

[maxitg]

Requires #579.

The problem

We currently build and test Wolfram Language code in the same Circle CI job, which results in awkward behavior such as artifacts being uploaded 4 times since tests are parallelized #578. We should split them.

Possible solution

This would be straightforward if not for the fact that downloading a docker image with Mathematica takes ~60 seconds which makes splitting the jobs more expensive. We need to implement #579 first.