tests: functional: add alert helpers
tests: functional: add alert helpers

The selenium functions to wait, accept and dismiss alert() popups
are not oneliners, wrap those into helpers.

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