flush the redis db before using it in a test
flush the redis db before using it in a test

there's already a warning in redis.yml.example that the test redis db
will be flushed during tests, but we missed one instance of doing so,
here

Change-Id: Iab9c4f4eae0779858b1fc9010e857e2f0db15a9a
Reviewed-on: https://gerrit.instructure.com/4706
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@instructure.com>
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

parbros/red-innovadores-lms,eCNAP/CANVAS,wimemx/Canvas,HotChalk/canvas-lms,nditech/red-innovadores-lms,roxolan/canvas-lms,instructure/canvas-lms,whalejasmine/canvas-lms,loganrun/pincan,juoni/canvas-lms,usmschuck/canvas,usmschuck/canvas,samuels410/greatlakes-lms,antoniuslin/canvas-lms,skmezanul/canvas-lms,efrj/canvas-lms,sli7246/canvas_branch,va7map/canvas-lms,utkarshx/canvas-lms,Jyaasa/canvas-lms,sli7246/canvas_branch,arrivu/hoodemo,dgynn/canvas-lms,jay3126/canvas-lms,snehakachroo1/LSI_replica,jay3126/canvas-lms,AndranikMarkosyan/lms,landonwilkins/canvas-lms,shidao-fm/canvas-lms,arrivu/arrivu-lms,greyhwndz/canvas-lms,antoniuslin/canvas-lms,samuels410/greatlakes-lms,em2/canvas-lms,va7map/canvas-lms,greyhwndz/canvas-lms,faraazkhan/canvas,wimemx/Canvas,redconfetti/canvas-lms,wye0220/shindig-canvas,sfu/canvas-lms,HotChalk/canvas-lms,sdeleon28/canvas-lms,ShreniiNepal/shrenii_lmp,arrivu/hiring-site,efrj/canvas-lms,grahamb/canvas-lms,Unow/ins,parbros/red-innovadores-lms,leftplusrightllc/canvas-lms,venturehive/canvas-lms,narigone/canvas,gbleydon/canvas-lms,merichar/canvas-lms,arrivu/hiring-site,sfu/canvas-lms,jay3126/canvas-lms,snehakachroo1/LSI_replica,juoni/canvas-lms,venturehive/canvas-lms,whalejasmine/canvas-lms,Rvor/canvas-lms,djbender/canvas-lms,shidao-fm/canvas-lms,djbender/canvas-lms,wrdsb/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,Unow/ins,tgroshon/canvas-lms,eCNAP/CANVAS,dgynn/canvas-lms,grahamb/canvas-lms,eCNAP/CANVAS,SwinburneOnline/canvas-lms,arrivu/qibla-ecampus,eCNAP/CANVAS,juoni/canvas-lms,parbros/red-innovadores-lms,loganrun/pincan,leftplusrightllc/canvas-lms,landonwilkins/canvas-lms,faraazkhan/canvas,narigone/canvas,dgynn/canvas-lms,instructure/canvas-lms,AndranikMarkosyan/lms,sfu/canvas-lms,rup/Tarrax,Jyaasa/canvas-lms,HotChalk/canvas-lms,fronteerio/canvas-lms,kyroskoh/canvas-lms,arrivu/qibla-ecampus,efrj/canvas-lms,usmschuck/canvas,landonwilkins/canvas-lms,roxolan/canvas-lms,matematikk-mooc/canvas-lms,greyhwndz/canvas-lms,antoniuslin/canvas-lms,usmschuck/canvas,arrivu/hiring-site,tgroshon/canvas-lms,va7map/canvas-lms,snehakachroo1/LSI_replica,redconfetti/canvas-lms,AndranikMarkosyan/lms,samuels410/greatlakes-lms,narigone/canvas,em2/canvas-lms,kyroskoh/canvas-lms,faraazkhan/canvas,redconfetti/canvas-lms,SwinburneOnline/canvas-lms,icelab/canvas,Unow/ins,sdeleon28/canvas-lms,tgroshon/canvas-lms,landonwilkins/canvas-lms,Jyaasa/canvas-lms,rup/Tarrax,wimemx/Canvas,gbleydon/canvas-lms,icelab/canvas,fronteerio/canvas-lms,AndranikMarkosyan/lms,arrivu/arrivu-lms,skmezanul/canvas-lms,roxolan/canvas-lms,arrivu/arrivu-lms,arrivu/jigsaw-lms,merichar/canvas-lms,Rvor/canvas-lms,kyroskoh/canvas-lms,instructure/canvas-lms,wrdsb/canvas-lms,djbender/canvas-lms,ottenhoff/canvas-lms,SwinburneOnline/canvas-lms,wrdsb/canvas-lms,shidao-fm/canvas-lms,instructure/canvas-lms,wye0220/shindig-canvas,rup/Tarrax,arrivu/hoodemo,ottenhoff/canvas-lms,sli7246/canvas_branch,sfu/canvas-lms,va7map/canvas-lms,samuels410/greatlakes-lms,jay3126/canvas-lms,roxolan/canvas-lms,leftplusrightllc/canvas-lms,Jyaasa/canvas-lms,arrivu/jigsaw-lms,shidao-fm/canvas-lms,antoniuslin/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,fronteerio/canvas-lms,ankithbti/canvas_lms,nditech/red-innovadores-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,ShreniiNepal/shrenii_lmp,Unow/canvas-lms,gbleydon/canvas-lms,venturehive/canvas-lms,nditech/red-innovadores-lms,sfu/canvas-lms,wye0220/shindig-canvas,leftplusrightllc/canvas-lms,ottenhoff/canvas-lms,instructure/canvas-lms,merichar/canvas-lms,Rvor/canvas-lms,whalejasmine/canvas-lms,utkarshx/canvas-lms,sdeleon28/canvas-lms,HotChalk/canvas-lms,dgynn/canvas-lms,greyhwndz/canvas-lms,wrdsb/canvas-lms,redconfetti/canvas-lms,em2/canvas-lms,Unow/canvas-lms,arrivu/hoodemo,instructure/canvas-lms,sdeleon28/canvas-lms,nditech/red-innovadores-lms,grahamb/canvas-lms,narigone/canvas,matematikk-mooc/canvas-lms,efrj/canvas-lms,ShreniiNepal/shrenii_lmp,grahamb/canvas-lms,matematikk-mooc/canvas-lms,djbender/canvas-lms,skmezanul/canvas-lms,instructure/canvas-lms,utkarshx/canvas-lms,Rvor/canvas-lms,arrivu/jigsaw-lms,whalejasmine/canvas-lms,tgroshon/canvas-lms,ankithbti/canvas_lms,icelab/canvas,loganrun/pincan,utkarshx/canvas-lms,skmezanul/canvas-lms,snehakachroo1/LSI_replica,ShreniiNepal/shrenii_lmp,kyroskoh/canvas-lms,ankithbti/canvas_lms,sfu/canvas-lms,juoni/canvas-lms,Unow/canvas-lms,arrivu/qibla-ecampus