chore: find a better more automatic way to distribute our tests

[erezrokah]

Follow up to #4164.

In #4164 we split our tests across multiple machines using an ava feature that allows it.
The feature works by sorting specs by file names, dividing those into chunks and sending each chunk to a different machine.
As a result of ava's distribution strategy and since some of our specs are slower than others, I manually distributed the tests using a naming scheme (prefixing with numbers).

See a suggestion in #4164 (comment).
Another possible option is to have a lint rule to enforce a maximum number of tests in a spec, to keep each spec execution time low

[erezrokah]

We could possibly contribute to https://github.com/avajs/eslint-plugin-ava

[erezrokah]

Opened avajs/ava#2965 for reference

[sarahetter]

Moving away from ava