proposal: runtime: Expose a function to enable GODEBUG="tracebackancestors=N" on runtime

[ysmood]

I'm making a lib to detect goroutine leaks, if we don't enable tracebackancestors it will be impossible to detect goroutine leaks in testing when testing.T.isParallel is enabled.

If we can add a function in runtime, such as runtime.TracebackAncestors(n int) then we don't have to add protection code like this:

https://github.com/ysmood/gotrace/blob/03c1fdd54f28e0778ae12534167858777a00ddcd/utils.go#L31-L38

[ianlancetaylor]

Perhaps we should have a general mechanism for setting GODEBUG values that are used by the runtime.

[gopherbot]

Change https://go.dev/cl/397517 mentions this issue: runtime: resolve #52093