Replaced some code in slide_tile_to_left with method call slide_single_to_left
Broke unit tests for slide_tile....:
non-1 value tiles shift left - FAILED: {2,0,2,0} became {2,0,2,0} instead of {2,2,0,0}
Alternate value tiles stay in order - FAILED: {2,0,2,8} became {2,0,2,8} instead of {2,2,8,0}
Value in (left) middle shifts to left edge after shift - FAILED: {2,0,2,0} became {2,0,2,0} instead of {2,2,0,0}