Add tests exercising signals and signal handlers

[jmid]

OCaml 5 has (had) issues with signals and signals handlers, e.g., https://github.com/ocaml/ocaml/issues/12253 and https://github.com/ocaml/ocaml/issues/13189 It would therefore make sense to enhance the test suite with a test exercising these runtime parts. Likely some combination of the following could be a good starting point:

• signals and handlers,
• allocations,
• plain computation (tak?)
• GC (implicitly or explicitly called?)
• both tail, non-tail, and C-calls

This was suggested by @talex5 who has by bitten by the above.