cmd/go: make go test build multiple executables

cmd/go: make go test build multiple executables

If -c is set while testing multiple packages, then allow
to build testing binary executables to the current directory
or to the directory that -o refers to.

$ go test -c -o /tmp ./pkg1 ./pkg2 ./pkg2
$ ls /tmp
pkg1.test pkg2.test pkg3.test

Fixes #15513.

Change-Id: I3aba01bebfa90e61e59276f2832d99c0d323b82e
Reviewed-on: https://go-review.googlesource.com/c/go/+/466397
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>