Comment 4 for bug 955053

Rather than doing this check at runtime, I added a simple test that validates that all jobs that have a user also use environ for all the variables referenced in the command.

This still leaves us uncovered in two cases:

1- A variable is referenced in a script rather than in the job file. This is bad practice as hardcoding environment variable names in scripts is too opaque.

2- User-written jobs. Since the tests work only on the jobs we ship with checkbox, anything outside that will not be tested.

Still, this will prevent us from shipping broken jobs in checkbox itself which I think is an improvement anyway.