Support for packages without host application.

[michael-zimmermann]

It is not possible to run accessibility snapshot testing (using point-free ios-snapshotting) when creating swift packages.
The snapshots crash with

AccessibilitySnapshot/SnapshotTesting+Accessibility.swift:57: Fatal error: Accessibility snapshot tests cannot be run in a test target without a host application

It doesn't appear that the host application is necessary, at least with the point-free framework. Here is a snapshot where the default guard causing the fatal error is overridden.

I propose, to be able to pass a variable to the snapshotting-strategy to avoid checking if a host application is available. The default value true of this new parameter would keep the old strategy working the way it did and cause no backwards compatibility problems. By orverriding this to false, accessibility snapshots could also be done for swift packages.

        assertSnapshot(
            of: view,
            as: .accessibilityImage(
                shouldRunInHostApplication: false
            )
        )