Fail when `--concurrency` is set to `0`

[novemberborn]

Inspired by #1463. --concurrency=0 currently means AVA spins up as many workers as there are test files, and when #1467 lands it means AVA spins up as many workers as there are CPU cores. In either case, we should refuse to run tests when the value is set to 0 since AVA's behavior won't match user expectations.

See a similar test in lib/cli.js:

ava/lib/cli.js

Line 121 in a0d5b37

if (cli.flags.concurrency === '') {

[mliou8]

hey @novemberborn . I read through the ticket and I'd like to try and make an attempt at it if that's all right.

[novemberborn]

Hey @mliou8, thank you for your interest. As it turns out @tdeschryver already took this on in #1478. I hope you find one of the other open issues interesting to work on though!

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In the end we figured to allow the 0 value. We now default to --concurrency mode, and the 0 value lets AVA pick a machine-appropriate number of worker processes. The flag should be able to override any package.json#ava value.