add test modules for support

this allow to test for construct_runtime as well.

The reason to have put that in another crate is:
* if we put test in `tests/` dir of srml/support then decl_storage fails to get
  srml-support access because it believes it is inside srml-support
  crate and so derive access to `quote!{ crate }` but this is wrong
  (and I don't see any way to prevent that, and it only bother us so I
  don't think that matters that much)
* if we put test inside lib.rs then contruct_runtime cannot be used
  because it call some macros that are defined with macros
  (decl_outer_event and decl_outer_origin) and thus rustc complains.