cmd/go: nicer error diagnosis in go test

cmd/go: nicer error diagnosis in go test
Before,
        go test -bench .
would just dump the long generic "go help" message. Confusing and
unhelpful. Now the message is short and on point and also reminds the
user about the oft-forgotten "go help testflag".

        % go test -bench
        go test: missing argument for flag bench
        run "go help test" or "go help testflag" for more information
        %

R=rsc
CC=golang-dev
https://golang.org/cl/12662046