test-middlend: do not require copying pass names in dune files

[nomeata]

This refines upon #194, which was quickly getting boring. This refines the setup so that the dune file does not need to know the names and order of passes.

Unfortunately, dune does not support comparing directories (ocaml/dune#3567), so this uses a self-updating dune include to work-around that, as suggested in
https://dune.readthedocs.io/en/stable/reference/dune/include.html

[nomeata]

(Not sure if the complexity of a dune include is justified here.)

[nomeata]

This may be not be the final solution (ocaml/dune#3567 would help), but it’s certainly an improvement that one doesn’t have to manually edit dune files, and that

dune runtest test-middlend --auto-promote --diff-command "diff -Nur"

can fix things up.

[nomeata]

@DCupello1 should we merge this before merging all the new passes? It does help resolving the merge conflicts if one doesn't have to manually edit the dune file (but merge conflicts still have to be resolved locally)

[DCupello1]

Happy to merge this in as well.

(looks like this is just making it so you don't have to copy i.e. "06-sub.il" many times around which obviously makes it easier for both of us)

[nomeata]

Yes, and after a merge one can run

dune runtest test-middlend --auto-promote

to update that file. Still not as convenient as it should be, but an improvement.

[nomeata]

Actually, let me add a .gitattribute to mark the generated file as binary, this way we never get conflict markers therein, which would confuse dune.