add .failed_tests to .gitignore
add .failed_tests to .gitignore

Add .failed_tests to the .gitignore file. Was an oversight that it was
not included in the PR responsible for implementing --failed-tests feature.

Refs #8509

backmari/moose,sapitts/moose,bwspenc/moose,Chuban/moose,milljm/moose,permcody/moose,sapitts/moose,laagesen/moose,lindsayad/moose,milljm/moose,lindsayad/moose,andrsd/moose,harterj/moose,idaholab/moose,nuclear-wizard/moose,permcody/moose,dschwen/moose,yipenggao/moose,dschwen/moose,bwspenc/moose,jessecarterMOOSE/moose,idaholab/moose,idaholab/moose,yipenggao/moose,harterj/moose,liuwenf/moose,harterj/moose,bwspenc/moose,jessecarterMOOSE/moose,SudiptaBiswas/moose,liuwenf/moose,YaqiWang/moose,YaqiWang/moose,Chuban/moose,SudiptaBiswas/moose,milljm/moose,idaholab/moose,permcody/moose,harterj/moose,sapitts/moose,bwspenc/moose,nuclear-wizard/moose,laagesen/moose,laagesen/moose,laagesen/moose,friedmud/moose,stimpsonsg/moose,backmari/moose,stimpsonsg/moose,jessecarterMOOSE/moose,nuclear-wizard/moose,milljm/moose,backmari/moose,liuwenf/moose,friedmud/moose,stimpsonsg/moose,lindsayad/moose,idaholab/moose,andrsd/moose,Chuban/moose,permcody/moose,yipenggao/moose,liuwenf/moose,yipenggao/moose,backmari/moose,SudiptaBiswas/moose,bwspenc/moose,laagesen/moose,Chuban/moose,harterj/moose,dschwen/moose,andrsd/moose,liuwenf/moose,YaqiWang/moose,friedmud/moose,jessecarterMOOSE/moose,dschwen/moose,sapitts/moose,lindsayad/moose,jessecarterMOOSE/moose,milljm/moose,lindsayad/moose,andrsd/moose,liuwenf/moose,YaqiWang/moose,nuclear-wizard/moose,SudiptaBiswas/moose,friedmud/moose,SudiptaBiswas/moose,dschwen/moose,sapitts/moose,andrsd/moose,stimpsonsg/moose