internal/pkgbits: add missing tests

The encoding version was recently bumped to V3
but the tests were not updated. Do it now.

For #9859.

Change-Id: Ia61d3e178215e804dcd2db186774883617e5e5a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/757440
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>