Feed mocha test duration as 'runDuration' to reporter
Feed mocha test duration as 'runDuration' to reporter

So that reporters get `runDuration`from mocha runner.

Similar to qunit https://github.com/testem/testem/commit/191ba59574e4a29b8d50923b02e673990e70b830
testem/testem,testem/testem,Addepar/testem,Addepar/testem,paulot/testem,paulot/testem,bradleylandis/testem,paulot/testem,Addepar/testem,fotinakis/testem,Addepar/testem,johanneswuerbach/testem,bradleylandis/testem,fotinakis/testem,bradleylandis/testem,fotinakis/testem,johanneswuerbach/testem,johanneswuerbach/testem,testem/testem