Tests: listen to end events on collectors and make sure they drain

Hello @evansolomon, 

Please review the following commits I made in branch 'es-test-listeners'.

0752df64e0f78abee5e929c0fe30a47309fe464d (2014-03-27 16:21:17 -0400)
Tests: listen to end events on collectors and make sure they drain

R=@evansolomon