errors/fmt: test fmt copy not std lib fmt
errors/fmt: test fmt copy not std lib fmt

This avoids breaking these tests when std lib fmt changes.

Change-Id: Ia88bc75b584076f0aa7b6a11245548b73abde3cd
Reviewed-on: https://go-review.googlesource.com/c/exp/+/249359
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/exp,golang/exp,golang/exp