Reduce the flakiness of TestPeach with some random jitter.
Reduce the flakiness of TestPeach with some random jitter.

There was a recent failure of the test that checks peach is
nondeterministic: https://github.com/elves/elvish/runs/2926941131

The failure can be reproduced reliably when setting GOMAXPROCS=1.
However, GitHub Action's Windows runner has two CPU cores, which means
GOMAXPROCS should be 2. Presumably, during that particular one, one of
the cores is occupied by another process, so Go had to run the
goroutines in order on one CPU core.

Go's time.Sleep yields the current goroutine and allow other scheduled
goroutines to run on the same thread. As a result, adding a random
jitter guarantees nondeterminism in execution order, even if only one
CPU core is available to execute threads.

Also run the nondeterminism check in an infinite loop.

elves/elvish,elves/elvish,elves/elvish,elves/elvish,elves/elvish