cmd/go: make 'go test -h' print two lines

Like every other command's -h flag. To achieve this, pass the command's
usage function to the cmdflag package, since that package is used by
multiple commands and cannot directly access *base.Command.

This also lets us get rid of testFlag1 and testFlag2, and instead have
contiguous raw strings for the test and testflag help docs.

Fixes #26999.

Change-Id: I2ebd66835ee61fa83270816a01fa312425224bb3
Reviewed-on: https://go-review.googlesource.com/c/144558


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>