Test scheduler interface improvement

[kwonoj]

I'm writing it as an issue based on discussions we had on core meeting -

Testscheduler included in core is working as expected, but also lacks of some ergonomics around setups and assertions - reason there are couple of variant for 3rd party assertion suite out there. I'd like to self-advertise to my own implementation to adapt into current schedules, which satisfies most of complaints around current testscheduler:

• frametime adjustment (1 by default, customizable)
• test framework agnostic
• long-timeframe support via extended marble syntax
• fine-grained time travelling (advanceTo interface as similar v4)
• visual friendly assertion helper function

It could be either make pkg itself as upstream core (once we have monorepo) or either cherrypicking necessary implementation only, or something else - notable thing is core (or separate monorepo-pkg from core) provides de-facto OOTO solution for testing to anyone can easily use.

My current implementation is available at https://github.com/kwonoj/rx-sandbox currently.

/cc @benlesh
(and also @jayphelps , heard you are looking for something similar? not sure but pinging)

[kwonoj]

afaik v6 have ongoing discussions for new test scheduler interface, but it has some core differences with proposed changes here. It'll be tracked separately.