tests: kill backgrounded processes more robustly

t0081 creates several background processes that write to a fifo and
then go to sleep for a while (so the reader of the fifo does not see
EOF).

Each background process is made in a curly-braced block in the shell,
and after we are done reading from the fifo, we use "kill $!" to kill
it off.

For a simple, single-command process, this works reliably and kills
the child sleep process. But for more complex commands like
"make_some_output && sleep", the results are less predictable. When
executing under bash, we end up with a subshell that gets killed by
the $! but leaves the sleep process still alive.

This is bad not only for process hygeine (we are leaving random sleep
processes to expire after a while), but also interacts badly with the
"prove" command. When prove executes a test, it does not realize the
test is done when it sees SIGCHLD, but rather waits until the test's
stdout pipe is closed. The orphaned sleep process may keep that pipe
open via test-lib's file descriptor 5, causing prove to hang for 100
seconds.

The solution is to explicitly use a subshell and to exec the final
sleep process, so that when we "kill $!" we get the process id of the
sleep process.

[jn: original patch by Jeff had some additional bits:

   1. Wrap the "kill" in a test_when_finished, since we want
      to clean up the process whether the test succeeds or not.

   2. The "kill" is part of our && chain for test success. It
      probably won't fail, but it can if the process has
      expired before we manage to kill it. So let's mark it
      as OK to fail.

 I'm postponing that for now.]

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

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