support splitting tests in CI

[fengmk2]

like ava https://github.com/avajs/ava/blob/main/docs/recipes/splitting-tests-ci.md

there is a way to work on mocha mochajs/mocha#4958

[fengmk2]

https://github.com/jamiebuilds/ci-parallel-vars 有一个库获取环境变量，然后实现代码参考 avajs/ava@c4f607c avajs/ava#1812