Add grace-period timeout handling for stack test suites

[DanBurton]

Please add an option to pair with --test-suite-timeout that does staged termination:

• on timeout, send SIGTERM
• wait configurable grace period (e.g. --test-suite-timeout-grace)
• if still running, send SIGKILL

This would give a per-test-suite guaranteed stop.

As a stackage curator, I want this because we use stack to run stackage nightly builds, and recently had a test suite get stuck in a way that was resistant to SIGTERM.

(Back-compat: if grace is not set, keep current behavior unchanged.)

Here is an example of how I implemented something like this in Haskell after a bit of trial and error (race was not cutting it), and a test that demonstrates that it works: commercialhaskell/curator@afbc5f3#diff-6e439c3b5af547403c42a51c36fc21f684cb244284ec796a0f8386d8785d7143

[mpilgrem]

Fixed in:

• --test-suite-timeout-grace #6868.