Engine: cleanup side effects between tests

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: builtin command 'commands' returns a list of registered commands

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4314 from shykes/engine-commands

Engine: builtin command 'commands' returns a list of registered commands