strings: add TestIndexByte
strings: add TestIndexByte

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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,cloudflare/go,josharian/go,akutz/go,akutz/go,CAFxX/go,josharian/go,josharian/go,golang/go,josharian/go,akutz/go,golang/go,golang/go,cloudflare/go,akutz/go,nawawi/go,golang/go,akutz/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,josharian/go,CAFxX/go,josharian/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,golang/go,akutz/go,nawawi/go,CAFxX/go,golang/go