We've seen some instability in the test, compare

  https://github.com/haskell/cabal/issues/8416

This adds timestamped output to make it easier to
see what goes wrong in case the test fails again.

This makes the `finally` apply to the last `do` block as intended.
No real effect on the test, though.

The "up and running" file was written before the signal handler
was installed, so it was possible for the test to kill the
subprocess before, leading to a missed "exiting" line in the
output.

I hope this fixes the sporadic failures we're seeing (#8080).