API and associated Runner logic for performing multiple repetitions of a test

[stmontgomery]

Description

This tracks making it possible to repeat a test in swift-testing multiple times. I imagine it would be best expressed as a Trait type, and would require associated logic in the Runner and Event subsystems.

rdar://92412320

[grynspan]

We had toyed with this before we shared the repo publicly. I think we ran into trouble because it wasn't clear what the fine semantics of a .repeating(n) trait would be.

[grynspan]

I think we've landed this as tools SPI?

[stmontgomery]

I think we've landed this as tools SPI?

This issue is tracking adding that ability on a per-test or -suite level, likely via a trait