Do not start tests after global timeout

Description / Motivation

Fixes #16 (closed)

How to Test the MR Manually

Duplicate the test in test/unix/test_process.ml (don't forget to change the title), then run:

dune exec test/unix/main.exe -- -k --global-timeout 0.5

Checklist

• Update CHANGES.md. No need to document changes to documentation and tests.
• Make sure all new values, types etc. are documented in .mli files and that the generated documentation looks ok.
• Add tests in test/, if relevant.