In golang.org/cl/74352, the print rules were overhauled to give better
error messages. This also meant adding a regex to find and extract the
used formatting verbs.

However, %v was missed. Add it to the expression, and add a test too.

Fixes #22847.

Change-Id: If117cc364db0cb91373742239b8a626c137642b0
Reviewed-on: https://go-review.googlesource.com/79455
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>