🚀 Feature: Option to run async tests in parallel on a single process

[kuba-orlik]

Is your feature request related to a problem or a nice-to-have?? Please describe.

Running tests in parallel with mocha adds the overhead of using and spawning separate workers, which makes it slower than it has to be in test suites based on a lot of I/O and async behavior.

Describe the solution you'd like
Option to run all the tests at once in a single worker. Think of a Promise.all for all the its

Describe alternatives you've considered
Not sure how to achieve that effect in any other way

[nopeless]

#4588 I made an issue similar to this but with detailed plans @kuba-orlik

[JoshuaKGoldberg]

Same as #4588 (comment), alas.