Summary:
Add some basic unit tests for extutil.runbgcommand().
This also changes the behavior to throw an OSError if we fail to execute the
process, instead of a subprocess.CalledProcessError. This matches the behavior
of the subprocess module when it fails to execute the command.
Test Plan: Ran the tests.
Reviewers: stash, quark
Reviewed By: quark
Subscribers: net-systems-diffs@fb.com, yogeshwer, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D4864999
Signature: t1:4864999:1491874918:c03edafe02af217e41c28a770137bfd72bcbba9b
