CTest normally terminates test processes on timeout using `SIGKILL`.
Offer tests a chance to exit gracefully, on platforms supporting POSIX
signals, by setting `TIMEOUT_SIGNAL_{NAME,GRACE_PERIOD}` properties.
Fixes: #17288
When a test's process creates its own child and exits, the grandchild
may keep pipes open. Fix CTest logic to correctly timeout if the
grandchild does not exit and close the pipes before the timeout expires.
This was broken by commit b5e21d7d2e (CTest: Re-implement test process
handling using libuv, 2017-12-10, v3.11.0-rc1~117^2) which added an
unnecessary condition to the timeout handling.
Fixes: #20116
