Run tests in parallel

[RReverser]

• Run tests in parallel. On my ThinkPad laptop with 8 cores, this reduces tests execution time from 60-70 seconds to 30-35 seconds (essentially ~2x faster turnaround).
• Move Mocha settings to a config file. This allows to easily invoke Mocha alone via npx mocha, or for e.g. VSCode Test Explorer to find & run tests with the correct settings automatically.

[lovell]

Thank you for this improvement Ingvar.

There are a few util tests (cache, concurrency, counters) that require sequential running and may sporadically fail or cause other tests to fail when run in parallel. We should ensure these run after other tests have completed, perhaps via some kind of "after all" hook.

[RReverser]

There are a few util tests (cache, concurrency, counters) that require sequential running and may sporadically fail or cause other tests to fail when run in parallel.

I actually did run into that in my ahem other branch, but not with native build, even though I tried even running those asserts in a tight loop while Mocha was executing other test files.

Note that Mocha's parallel mode only parallelises tests at file-level, rather than per-test level, so I guess what happens is that each individual test file gets clean environment with its own copy of native module anyway, hence the counters are indeed zero in the individual test.

[RReverser]

If I try to add e.g. console.log("sharp init") at the top level of constructor.js, I also see number of logs corresponding to my number of CPUs:

sharp init
sharp init
sharp init
sharp init
sharp init
sharp init
sharp init

So it does reuse the instances, but within each instance tests are running one by one and can't step on each others' toes and concurrently modify globals - that's why counters test passes successfully.

[lovell]

Great, it looks like the overall situation has improved since I last tried this, e.g. commits c4bcd08 and d7d580a

Let's merge and should we experience flaky tests we can deal with specific failures, if any. Thanks again!

[RReverser]

No worries, and yeah, it's certainly a valid concern and something I tried to test extensively before making the PR. And I'm still not 100% sure - parallel stuff is always hard 😅 Happy to make another one if something does turn out flaky.