esm: improve performance & tidy tests

This standardises the setup of ESM tests and improves performance by parallelising case runs using a promise wrapper around async spawn.

I originally started out using chaining for parallelisation, but switched to node:test to avoid a bajillion .then()s. As .then()s, the performance improvement was obvious.

I'm not sure how to get a truly apples-to-apples comparison: ./tools/test.py -J es-module doesn't benefit from node:test's parallelisation. I manually checked one of the largest tests, and the improvement is quite stark:

$> time ./node ./test/es-module/test-esm-loader-chaining.mjs

When	Duration
Before	0m1.168s
After	0m0.442s

Note to reviewers: enabling Hide whitespace reduces the diff by about a third.

cc @nodejs/loaders