Test multiple packages with ... wildcard

The "..." wildcard allows "go test" to test multiple sub packages, however this fails with "gopherjs test":

```
$ gopherjs test -v github.com/foo/bar/...
cannot find package "github.com/foo/bar/..." in any of:
    /usr/local/go/src/github.com/foo/bar/... (from $GOROOT)
    /Users/dave/dev/src/github.com/foo/bar/... (from $GOPATH)
```

It would be great to support this feature.
