Add TestIndexByte to package strings similar to the already existing
TestIndexByte in package bytes.

Change-Id: Ib60695cb326156a4fe48138c66393ebbd11e4a25
Reviewed-on: https://go-review.googlesource.com/c/go/+/171197
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>