flag: make tests silent

A few of the tests were printing garbage to stderr,
since FlagSet's default Output is os.Stderr:

	$ go test
	flag provided but not defined: -x
	invalid value "1" for flag -v: test error
	Usage of test:
	flag needs an argument: -b
	Usage of test:
	  -b	usage
	PASS
	ok  	flag	0.008s

Add the remaining SetOutput(io.Discard) method calls.

Note that TestUserDefinedFunc was a tricky one.
Even with the added SetOutput calls,
the last part of the test would still print usage text to stderr.
It took me a while to figure out the problem was copying FlagSet.
I've filed go.dev/issue/51507 to record this particular sharp edge,
and the test code now avoids making FlagSet copies to avoid the bug.

Change-Id: I323f24091b98386312aa72df3eb890af6625628d
Reviewed-on: https://go-review.googlesource.com/c/go/+/390234


Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>