Another testing issue with signatures

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

Something just occurred to me: we also need a way to write tests *inside* a 
module, rather than the current model where the tests are run as external 
Mirage programs.

The reason for this is that we restrict the external interfaces to the safe 
signatures (e.g. abstract type t), and so the only place the test can compile 
is within the implementation itself.

Or (a bit more radically), we could compile up a 'naked Mirage' with none of 
the MLI files present, which should be strictly more expressive, and simply run 
the tests externally as we currently do.  That should be reasonably easy to do 
as the only place we actually compile anything is in _build/<backend>/std 
(libraries are source copied in there)...

Anil