testing: streamline logic in loopSlowPath
There's a fair amount of duplication of logic between various return
branches of loopSlowPath and stopOrScaleBLoop. Restructure these so
there's a single "keep going" path and a single "we're done" path.
Change-Id: I38e4c7a616f8bd7707f3ca886f38ff21dbd78b6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/659658
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>