3346: Add JS test to check JS disabled is false
3346: Add JS test to check JS disabled is false

There should be a hidden input on the page which initially has its value
set to true. The JS should immediately set this to false so that we know
the user has JS enabled.

Authors: da39a3ee5e6b4b0d3255bfef95601890afd80709@richardtowers

alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend