secure/precis: fix test failure on Unicode version skew

Skip runes that do exist yet in the version of Unicode
used in secure/precis, even if they exist in the
Unicode version used in core.

Issue golang/go#16234

Change-Id: If75ce2b752c3ae0de9cde6937b613d5874149a74
Reviewed-on: https://go-review.googlesource.com/24691
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
1 file changed