The test for the race detector itself had a race of a sort not
detected by the race detector.

Fixes #18286.

Change-Id: I3265eae275aaa2869a6b6d3e8675b0d88b25831b
Reviewed-on: https://go-review.googlesource.com/34287Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>