The test was measuring something, assuming other goroutines had
already scheduled.

Fixes #10427

Change-Id: I2a4d3906f9d4b5ea44b57d972e303bbe2b0b1cde
Reviewed-on: https://go-review.googlesource.com/9561Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>