regenerate test files (./regenerate.bash)

This is required for the previous two commits (5a1e7ce and 3c53b4e).

Unfortunately, copybara (the tool we use to export this repository)
does not currently support re-generating .pb.go files for us.

Related to https://github.com/golang/open2opaque/issues/18

Change-Id: I4a25e8752e30623b5731fd4331b1e95dac32c96f
Reviewed-on: https://go-review.googlesource.com/c/open2opaque/+/699075
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>