bytes: add a boundary test for Count of 1 byte

bytes: add a boundary test for Count of 1 byte

This bottoms out in internal/bytealg.Count, which does a fair amount
of avx shenanigans. Make sure we're not reading out of bounds.

Change-Id: Ied0e461f536f75acc24c6797f7fc74e3f3901c10
Reviewed-on: https://go-review.googlesource.com/c/go/+/533116
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Achille Roussel <achille.roussel@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>