fmt: clear flags before printing extra argument errors

fmt: clear flags before printing extra argument errors

Do a reset of the fmt flags before printing the extra argument
error message to prevent a malformed printing of extra arguments.

Regroup tests for extra argument error strings.

Change-Id: Ifd97f5ca36f6c97ed5a380d975cf154d17997d3f
Reviewed-on: https://go-review.googlesource.com/20571
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>