sim: Add dependency tests to simulator

Create a trait `Depender` which is passed down to the image create and
verification to check and test for various types of dependency
resolution.  Add a test that uses this to test the simple case of unmet
dependencies preventing an upgrade.

The actual test is disabled (with an `if false ...`) because the code
under test loops forever in this configuration.

Signed-off-by: David Brown <david.brown@linaro.org>