cmd/go: allow arguments to 'go test' and 'go vet' to duplicate or override flags from GOFLAGS

This is a minimal fix for Go 1.14, but this parsing logic is much too
complex and seems like it will cause more trouble going forward.

I intend to mail a followup change to refactor this logic for 1.15.

Updates #32471

Change-Id: I00ed07dcf3a23c9cd4ffa8cf764921fb5c18bcd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/210940


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>