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 <bcmills@google.com>

errors/fmt/fmt_test.go

@@ -6,7 +6,7 @@ package fmt_test
 
 import (
 	"bytes"
-	. "fmt"
+	. "golang.org/x/exp/errors/fmt"
 	"io"
 	"math"
 	"reflect"