test: show limitation installing multi-context libs

[anmonteiro]

this test describes the limitation I mentioned in #10471 (comment).

our expectation is that "universal" libraries (which are defined in multiple contexts):

• can be installable next to native libraries
• automatically with dune install {PACKAGE_NAME}

this might require:

• checking that there are no collisions between the libraries (1 is exclusively (modes melange) the other doesn't contain (modes melange)
  • this is only a problem if libraries within the package share a public_name