Use old mocha exit behavior for test:diff
Use old mocha exit behavior for test:diff

As of Mocha 4.0, "Mocha will no longer force the process to exit once all
tests complete."  This behavior causes the test:diff script to hang.  It
can be suppressed by providing the --exit flag when invoking mocha, which
is done here for the test:diff script in package.json.

Bug: T196002
Change-Id: I92e6a3c54916f1b1c41e9ff32b2d7041cbe39b3f

wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps,wikimedia/mediawiki-services-mobileapps