Add Enaml regression tests.

Enaml does not define tests, but on the other hand we need to make sure that the core content remains stable for our applications. This PR defines a new sub-package, `enaml_tests`, where we can store regression tests against `enaml` itself.

The first test defined here tests that the `destroy` method in QtToolkitObject deletes the Qt widget. This currently fails, and I filed a PR in Enaml to fix it.

Please only merge when Enaml is fixed.