Add failing test when cross-compiling with ppxs

[TheLortex]

Fails with:

File "lib/dune", line 3, characters 18-24:
3 |  (preprocess (pps fooppx)))
                      ^^^^^^
Error: Library "fooppx" in _build/cross-environment/ppx is hidden
(unsatisfied 'enabled_if').
-> required by lib/lib.pp.ml (context cross-environment)
-> required by alias lib/all (context cross-environment)
-> required by alias default (context cross-environment)
Hint: try:
  dune external-lib-deps --missing --debug-dependency-path @@default
[1]

[rgrinberg]

Does this PR fix this issue? #3751

[TheLortex]

Hey, unfortunately no it doesn't.
It looks like in dune_rules/preprocessing.ml, calls to Lib.DB.resolve_pps are made with the wrong scope (using the current scope instead of the host scope).

[rgrinberg]

Did you forget to promote?

This test also needs a description of the problem that it's trying to test.

[TheLortex]

Promoted the error and added a description!