Register built-in engine commands at runtime instead of compile-time

This allows selective loading of commands, and paves the way to dynamic
plugins.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Cleanup integration tests

Refactor integration tests to remove special cases in the creation of
test engines. All test engines are now created through newTestEngine.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4249 from shykes/builtins

Register built-in engine commands at runtime instead of compile-time