This sets up a framework for running tests in parallel.
It increases the total time taken (since some steps are
repeated) but reduces the turn-around time significantly
overall.
The main objective is to make it possible to release more
test batches to grist-core without bringing CI to a crawl.
The clever little test/split-test.js script is from the
Grist Labs mono-repo and is Dmitry's work.
I considered doing the build in one job, and copying
it to test jobs, since it feels wasteful to repeat it.
That may be worth trying, especially if we start getting
jobs backing up (total concurrent Linux jobs on free plan
is quoted at 20).
It might also be worth looking at doing some tests in
parallel on the same worker, perhaps using the relatively
new MOCHA_WORKER_ID feature, since the tests are often not
actually CPU or I/O bound.
Paul Fitzpatrick
4be4acca6e