cmd/go: refer to testflag help in go test -help output
The change makes it easier for a user to get to the page where
she can check supported test flags, by adding 'go test testflag'
reference to the 'go test -help' output.
Fix #30365
Change-Id: I5b3db7853021ef68d096dcb467d7957d7e1bf623
GitHub-Last-Rev:
ce3dec59fcae0cca232372f01cdda98773c290c0
GitHub-Pull-Request: golang/go#30420
Reviewed-on: https://go-review.googlesource.com/c/163858
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>