Test postgres multi-schema introspection (#3297)

Test postgres multi-schema introspection

The test expectations show that there are cases where our choice _not_
to rename models and enums in case of conflict leads to ambiguous
situations that may be difficult for users to resolve. The test case
where this is the most apparent is
`multiple_schemas_w_duplicate_enums_are_introspected`.

closes prisma/prisma#15800