Move ios context specs

The iOS test suite has two different tests for contexts, one using the test app, and the other using the webview app. The latter includes all the tests in the former.

This removes the test app test, and promotes the webview one to the main directory for iOS (and fixes the tests up a little bit), in a first step toward organizing things functionally.

Also toward that end I've created the rudiments of a central desired capability module.