Updates xvfb config tests for Xenial support
Updates xvfb config tests for Xenial support

Separating tests into Trusty/Xenial versions, returning true if the
non-targeted platform is detected. We can't skip the test based on
platform info, because the test must execute in order to collect
platform info.

Also took the opportunity to move to the new "host" pragma for
testinfra, since the explicit module names format has been deprecated.

heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop