[Tests] Try to detect & use more threads in local

[PeterDaveHello]

[PeterDaveHello]

@ljharb is there anything else I can do here? Thanks.

[ljharb]

I'm basically just not convinced that the performance of local test runs is at all important compared to the complexity of deviating from travis (which is the source of truth).

[PeterDaveHello]

Basically it increased the complexity, that's true, but it'll help a lot to speed up the whole test process locally, especially the tests on local are not like the tests on Travis CI would be spreaded parallely, that's the point I think it'll help a lot!

Please let me know if I need to close this PR or you'd like to have more discussion or you'd like to get it merged, thanks a lot!

[PeterDaveHello]

@ljharb do you have any further suggestions? I think once we figure out how to properly run all the tests locally, it'll help to speed a lot, especially many people have more than 2 or even 4 cores CPU right now. Thanks for your time.