Prevent zombie process for `gem open`

The open command did not detach the editor process which meant it could
be zombied.  This should not be a big deal because the command exits
afterward but it doesn't hurt to clean up after ourselves.

See @4a3439c
Silence open command tests

The open command used `echo` as the test editor which would result in
the filename showing up in the output.  This is undesirable as the child
process output cannot be captured.

Now a command that does not produce output is used.