Bug 863946: mozrunner: support FreeBSD

Running `cfx test` fails on FreeBSD because it can't find the
application binary.  Specifying the application binary results in no
activity or output.

Recognise FreeBSD in ``sys.platform`` and look for the "firefox"
binary on FreeBSD.
Merge pull request #956 from frasertweedale/fix/cfx-freebsd-support

fix Bug 863946: mozrunner: support FreeBSD r=@ZER0