Add a test for promoting from a nonexistent subdir

[cpitclaudel]

This crashes on dune 2.5, saying this:

Error: exception Sys_error("x/y: No such file or directory")
Raised by primitive operation at file "stdlib.ml", line 324, characters 29-55
Called from file "src/stdune/io.ml", line 173, characters 8-126
Re-raised at file "src/stdune/io.ml", line 178, characters 8-23
Called from file "src/dune/artifact_substitution.ml", line 407, characters
  15-48
Called from file "src/fiber/fiber.ml", line 229, characters 16-19
Called from file "src/dune/build_system.ml", line 1588, characters 8-1023
Called from file "src/fiber/fiber.ml", line 114, characters 10-15
Re-raised at file "src/stdune/exn.ml" (inlined), line 37, characters 27-56
Called from file "src/stdune/exn_with_backtrace.ml", line 13, characters
  33-71
Called from file "src/fiber/fiber.ml", line 85, characters 10-17
Re-raised at file "src/stdune/exn.ml" (inlined), line 37, characters 27-56
Called from file "src/stdune/exn_with_backtrace.ml", line 13, characters
  33-71
Called from file "src/fiber/fiber.ml", line 85, characters 10-17

I must not crash.  Uncertainty is the mind-killer. Exceptions are the
little-death that brings total obliteration.  I will fully express my cases. 
Execution will pass over me and through me.  And when it has gone past, I
will unwind the stack along its path.  Where the cases are handled there will
be nothing.  Only I will remain.

On master it says Error: x/y: No such file or directory, which is cleaner but not great either. Ideally, I'd expect dune to just create the directory.

My use case is that I have many small tests in a single folder (let's say input1 … input50), and a single generated dune file saying how to generate a folder-full of outputs for each (let's say output/input1/xyz, … output/input50/xyz). This is committed in a git repo, and the output/input1 … output/inputn directories don't exist in the repo; but I do want to be able to create them by just running dune.

[ghost]

Are you trying to have targets in sub-directories? That's something we eventually want to support, but it's not supported yet.

[rgrinberg]

Updated in #6222