spanner/spannertest: make BYTES comparable
spanner/spannertest: make BYTES comparable

This is necessary for ORDER BY or GROUP BY on a BYTES-typed expression.

Change-Id: I29a740955ff9c447926fefec7e22a5272f772928
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/52894
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Knut Olav Løite <62634d14bf6dff6edb4b7af974862bb227095f71@gmail.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go