melange: add test to show flags passed from env

[jchavarri]

Fixes #6572. The PR just adds a test to demonstrate the functionality, but the implementation was added in #6569 inadvertently, as Dune_env and Dune_file both decode using the same function in Ocaml_flags:

dune/src/dune_rules/dune_env.ml

Line 191 in 1798cc6

let+ flags = Ocaml_flags.Spec.decode

dune/src/dune_rules/dune_file.ml

Line 172 in 1798cc6

and+ flags = Ocaml_flags.Spec.decode

Unless I'm missing something.

[rgrinberg]

I believe that it works, but how does this test verify that? How could I tell the flags are being passed?

[jchavarri]

I believe that it works, but how does this test verify that? How could I tell the flags are being passed?

Sorry, I am not sure what you mean. I refactored the test to proceed in 2 steps: first show the failing case, and then the fix using env, so the behavior becomes more obvious. Please let me know if this clarifies it or there is something else I'm missing.