Change "I click button" step to allow finding button by name attribute
Change "I click button" step to allow finding button by name attribute

As part of migrating to our apps to use GOV.UK Frontend buttons in forms
are going to change from `input` elements to `button` elements, which
breaks our tests because they don't currently search for buttons by name
attribute.

Currently some of our tests find a submit input by name attribute; this
commit simply changes the find xpath expression to include buttons with
a given value of the name attribute, to mirror how we can find inputs.
While this isn't best practice it is necessary because PhantomJS doesn't
take account of visibility well.

alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests