ctest: Set close-on-exec for jobserver FIFO

Set the close-on-exec flag to prevent the jobserver FIFO file
descriptor from leaking to subprocesses spawned by `ctest`.

This leak is usually harmless, but can cause `make test -jN` to hang
at the end of the build if a test leaves a background process running
with the jobserver FIFO file descriptor open.

Fixes: #26027