bytes, strings: use "reports whether" in HasPrefix and HasSuffix

bytes, strings: use "reports whether" in HasPrefix and HasSuffix

Update the doc comments to use the more idiomatic and common phrase
"reports whether" instead of "tests whether".

Change-Id: I2b7f8cce2d192f66e296ebaa9b37f37e8276b4ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/524898
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>