dune subst test for directory containing just a .opam file

[mtelvers]

PR supporting #11290

[Leonidas-from-XIV]

Can you make the tests succeed in CI? It's ok to add error output as long as it is reproducible.

(If you rebase the 5.3.0 CI failure should resolve itself, we updated the GH action since)

[mtelvers]

I have rebased this PR. I am working around the issue in OCaml-CI by adding this line to the build. However, it isn't perfect as, it sometimes creates a dune-project file where there wasn't one previously.

echo '(lang dune 3.0)' > './dune-project'

The error which I see is this:

#=== ERROR while compiling current.dev ========================================#
# context     2.3.0 | linux/arm64 | ocaml-base-compiler.4.14.2 | pinned(file:///src/ocurrent)
# path        ~/.opam/4.14/.opam-switch/build/current.dev
# command     ~/.opam/4.14/bin/dune subst
# exit-code   1
# env-file    ~/.opam/log/current-1-e2b7be.env
# output-file ~/.opam/log/current-1-e2b7be.out
### output ###
# File ".", line 1, characters 0-0:
# Error: There is no dune-project file in the current directory, please add one
# with a (name <name>) field in it.
# Hint: 'dune subst' must be executed from the root of the project.

The full log is here