Testing any value containing a Process.Id

The issue is similar to this one: https://github.com/elm-explorations/test/issues/24

We have a model containing a `Process.Id` (so that we can kill/restart it), but there is no way to create one for testing, since the `Platform` module don't export the constructor, which makes perfectly sense when you target production code, but not in the context of testing.

A very generic solution to this kind of problems would be to add conditional compilation, so that instead of:

```elm
module Platform exposing
( Program
, ...
, ProcessId
, ...
)
```

We could write:
```elm
module Platform exposing
( Program
, ...
# if env == test then
, ProcessId(..)
# else
, ProcessId
# end
, ...
)
```

Notice that this would solve a lot of other problems (like #24, `Debug.log` calls, etc...).