cmd/go: fix 'go vet -h' to print the right text

For the last two releases, its output has been the same as 'go -h'.

The test and vet sub-commands share their flag logic via the cmdflag
package, so fixing it there would mean a larger refactor. Moreover, the
test subcommand handles its '-h' flag in a special way; that's #26999.

For now, use a much less invasive fix, mirroring the special-casing of
'test -h' to simply print vet's short usage text.

Also add a regression test via a cmd/go test script.

Fixes #26998.

Change-Id: Ie6b866d98116a1bc5f84a204e1c9f1c2f6b48bff
Reviewed-on: https://go-review.googlesource.com/129318


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>