Add some tests for profile behaviour

Ordering: profiles are applied in order (from the active profiles
list in the Environment), with the last one winning as far as
property values goes. This *does* mean that a profile activated
inside application.yml is applied last and hence takes precedence.
It's debatable whether that is the right semantics, but that's
what it is for now.

Re gh-342: a profile added via SpringApplication also takes
precedence over one added on the command line. Also debatable
but at least it's predictable.

Naming: a profile adds "#<profile>" to the end of a property source
name (no more, no less)