Don't do a substring search to test for a timeout error.

Fixes #14722 (maybe)

Change-Id: I4e18c749d6fd92c084a1b0b83a805119e1ae5ff2
Reviewed-on: https://go-review.googlesource.com/20403
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>