-test flag is a testing only flag that enables all vet checks. It was needed
because there was no way to run all vet checks in a single command
invocation. However it is possible to do this now by combining -all and -shadow
flags.

Also a recently added -tests flag is similarly named, having both -test and
-tests can be confusing.

Change-Id: Ie5bacbe0bef5c8409eeace46f16141fa4e782c32
Reviewed-on: https://go-review.googlesource.com/20006Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>