Fixes#6673
The Meteor "dev bundle bin commands" which proxy through to the
meteor version of npm/node was not returning the exit code from the
command which it executed. This creates problems for things like
`meteor npm run script-name` when the exit code is important. This
comes into play when you run npm scripts which run tests, lint code, etc.
This fix causes the meteor-tool to process.exit with the spawned process
exit code.
Windows Disclaimer: I used the same flush-buffers-on-exit-in-windows
that the tool/cli/main.js uses because I would assume the same problem
exists, however, I don't have the Windows environment to test or confirm
that this code works at all.
Also, couldn't find any tests that directly tested this dev bundle
bin-command passing scenario (though hard to search through them all),
so I created a barebones test app and tests.
