tests: pages-layout: helper to save the alert text
tests: pages-layout: helper to save the alert text

It is meaningless to take a screenshot of an alert since it cannot be
customized. Provide a helper to save its content into a text file
instead. It is convenient to visually verify it has been localized as
expected.

garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,micahflee/securedrop,conorsch/securedrop,heartsucker/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,garrettr/securedrop,heartsucker/securedrop,micahflee/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,conorsch/securedrop,heartsucker/securedrop,micahflee/securedrop