Add another trunk prefix unit test.

I recently learned that trunk prefixes are not necessarily the digit 0.
In Russia, an 8 is used instead. This CL adds a unit test verifying that
libphonenumber covers such cases.

Bug: 1311937
Change-Id: I245a2400f120161ac930ab705dd89aacc823e223
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3755222
Reviewed-by: Matthias Körber <koerber@google.com>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1023606}