add reproduction for regression with exec + watch

[tatchi]

This demonstrates a regression when calling the `exec' command in watch mode from a child folder of the root folder.

So in short the following works fine

dune exec --watch /path/to/bin/main.exe

But not:

cd path && dune exec --watch ./to/bin/main.exe

In this case I get this error on Linux:

execve(_build/default/path/to/bin/main.exe): : No such file or directory

On MacOS, and according to the regression test, the error is different:

Error: posix_spawn(): No such file or directory

[tatchi]

Nothing to do with this PR, this is just a change that was never promoted

[tatchi]

We could probably come up with something better than sleep, but my point was to show the problem

[tatchi]

You can see the error in Linux on CI: https://github.com/ocaml/dune/actions/runs/8545271412/job/23413184499?pr=10380#step:11:26

[tatchi]

Reverting #10262 seems to fix the issue cc @gridbugs

[gridbugs]

Investigating...

[gridbugs]

Fixed in #10386. The problem was caused by relative paths to executables in the _build directory being resolved relative to the user's cwd rather than the project root. See the the commit message of 5038bbc for more info.

I've squashed your commits into a single commit with your signature attached and included it in my PR, mostly to add the tests you wrote.

Thanks for the report!

[tatchi]

Thanks for the quick fix @gridbugs 🤗