fmt: refined tests for non-string error verbs

This is a refinement of CL 164557.

Make it explicit in tests that using a non-string verb with
fmtError does not result in falling back to using fmt.Formatter.

Change-Id: I6d090f31818eb7cc7668d7565b1449c91cd03a23
Reviewed-on: https://go-review.googlesource.com/c/go/+/164701


Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>