Use more processors when running tests
Use more processors when running tests

Back on my old 2013 Macbook Pro, 2 was the optimum number of cores for the fastest test runs. Which is why I chose `-n2` when I originally introduced this flag (see 9ced677ec7547ae23aea589983aa13c1d50d4fec).

Now I have a faster Macbook (same as every other developer), which has more cores to take advantage of.

We also use 4 cores when running the API tests.

Cores | Time taken to run tests
--- | ---
1 | 28.31s
2 | 18.15s
3 | 14.33s
4 | 12.32s
5 | 12.98s
6 | 13.49s
8 | 15.37s
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