Accessibility updates for quiz show page

Corrected any noticed accessibility issues for quiz show control
buttons.

Closes: CNVS-6764

Test-Plan:
  - Test that quiz show buttons are buttons
  - Test that screenreaders read the buttons properly

Change-Id: I860b911e123500401d7925195a652918e56a33a1
Reviewed-on: https://gerrit.instructure.com/42106
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Ryan Taylor <rtaylor@instructure.com>