Isomorphic parser/printer

[kit-ty-kate]

Similarly to ocaml/opam#6506, opam-file-format doesn't respect the equation printer (parser x) = x.
For example, things like:

build: []

is simply discarded by the printer. See:

opam-file-format/src/opamPrinter.ml

Line 463 in 976d8ec

| Variable (_, { pelem = List { pelem = []; _}; _}) -> ()

This is the reason why this warning exists, as the internal reftest printer will simply remove the lines that the test is checking, making the test useless.