Flush output buffers on SIGTERM

We're currently using dune runtest in OCaml-CI as the standard way of running tests for a package. If some of the tests for a package get stuck in a deadlock, the CI job will kill the process after 60 minutes. Unfortunately, this means that the logs of the stalled test run are lost.

We could switch to using dune runtest --no-buffer in the CI, but this creates less readable (interleaved) test output. One possible alternative would be for Dune to flush its output buffers before responding to SIGTERM (probably keeping the old behaviour for SIGKILL). What do you think?