Feature request: Add the ability to specify at the test creation time the snapshot location

[aryx]

Right now the snapshots are stored in a cryptic directory (e.g., tests/snapshots/6ag6fdedfr/) which makes it hard
when reviewing PRs with tests to know to which test a snapshot expected output correspond to.

It would be good to have the snapshot and the test in the same directory so one could write

Test.create "testing tests/target/c/foo.c" ~checked_output:Testo.Stdout("tests/target/c/foo.stdout") (fun () ->
. ..
)

I think @neuroo was recently complaining about that.

[mjambon]

I think it would easy to add to the current implementation. I don't see a real downside.