The parcel scene in Oak's Lab plays Music_MeetRival on both the rival's
arrival and departure (lines 144-146 in oaks_lab.lua), but the post-battle
onStep exit sequence only played the fanfare when the rival approached
(fixed in #596). It was missing when the rival walks out after the battle.
Add stop_music + play_music Music_MeetRival before the rival's exit
walk-out in both oaks_lab.lua and oaks_lab_yellow.lua, matching the
parcel scene's double-fanfare pattern from the original ROM.
Fixes#683
bryanthaboi
Shane McGovern