Validate keywords that combine schemas with various logical operators.
Also add iterators over schemas. Now we only need them to skip some
tests, but later they will come in handy for non-test code.
Change-Id: Idaf41efea4d693b6be0ee5c6c3a527664b8c472f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668115
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
