cmd/guru: fix and reenable failing test case

And avoid assumptions about the contents of the standard hash/fnv
package.

Change-Id: I10cc95ac6e3b482da5b027d68ff218d08f425e89
Reviewed-on: https://go-review.googlesource.com/40870
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed