testutil: add iterator testing functions
testutil: add iterator testing functions

These functions test standard iterators.

Change-Id: I1ac442552a835f62773bcfc4acc4c06e0ec1d0f6
Reviewed-on: https://code-review.googlesource.com/5180
Reviewed-by: Michael McGreevy <e70fec140fa6773c6cfd45cb9b436bd6334f4c24@golang.org>

michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go