Execute all files in single process concurrently

[link89]

ava only execute one file in one process. This is good for isolation, but I hope that ava can have an option to allow executing all files in single process, and believe that users will do concurrency control by themselves.

This is good for io intensive cases. In our case, we have 100+ test files with an average 5 tests per file. Our test is based on selenium and we have a browser pool that allows open 50 browsers at the same time.

If we use ava -c 10 then we have to use distributed lock to ensure not more than 50 browsers are opened at the same time, or else case will failed, which is too complicated from my point of view.

If we use ava -c 1 then we cannot make full use of our browser pool because we don't have many cases in a file.

An idea solution for this scenario is provide an option to allow execute all files at the same time in a process. Then we can simply use tools like https://www.npmjs.com/package/throat instead of distributed lock for concurrency control. And since it is an io intensive task, so single process is acceptable.

[sindresorhus]

See #1332

[novemberborn]

(I'm closing this issue for housekeeping purposes, but let's keep the conversation going.)