Testruns are failing on OS X if real binary or new signed app bundle get specified (#183)

This will fix issue #183, and allows us to run a real binary and the new signed builds on OS X. @andreeamatei please review ASAP. Also can you please test this patch on Linux and Windows? I would kinda appreciate that.
Interesting that we need this since AFAIK mozmill does handle this internally...
(or at least should): as `mozmill -t %test% -b Firefox.app` works