refactor: in TestJumpTo, say 'sequence' instead of 'section'
refactor: in TestJumpTo, say 'sequence' instead of 'section'

'section' is a very outdated (<2012) way to refer sequences,
which we also call 'subsections'. The old terminology still exists
throughout the courseware djangoapps, but I decided to clean up this
one test case while I'm working on it.

edx/edx-platform,EDUlib/edx-platform,eduNEXT/edx-platform,angelapper/edx-platform,eduNEXT/edunext-platform,eduNEXT/edunext-platform,angelapper/edx-platform,arbrandes/edx-platform,edx/edx-platform,eduNEXT/edunext-platform,angelapper/edx-platform,eduNEXT/edx-platform,EDUlib/edx-platform,eduNEXT/edunext-platform,EDUlib/edx-platform,arbrandes/edx-platform,arbrandes/edx-platform,eduNEXT/edx-platform,edx/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,EDUlib/edx-platform,edx/edx-platform