Tests for 'wait' command: Improve reliability

I was seeing intermittent failures on CentOS5 until I made these
changes.

The usleep() calls should go between receiving a job and deleting it.
Improve the assertion error messages
Merge add-cmd-wait-for-job (#3) into ifixit-production

Tests for 'wait' command: Improve reliability

I was seeing intermittent failures on CentOS5 until I made these
changes.

The usleep() calls should go between receiving a job and deleting it.
Improve the assertion error messages