tests: cleanup background processes upon interruption

Reap background processes so that:
- Stray processes aren't left on the system
- Files aren't held open causing deletion issues on NFS
- Partitions used to run the tests from can be unmounted

* tests/tail-2/F-vs-missing.sh: Add the `kill && wait` of the
background $pid(s) to cleanup_().
* tests/tail-2/F-vs-rename.sh: Likewise.
* tests/tail-2/f-vs-rename.sh: Likewise.
* tests/tail-2/append-only.sh: Likewise.
* tests/tail-2/assert-2.sh: Likewise.
* tests/tail-2/assert.sh: Likewise.
* tests/tail-2/flush-initial.sh: Likewise.
* tests/tail-2/inotify-hash-abuse.sh: Likewise.
* tests/tail-2/inotify-hash-abuse2.sh: Likewise.
* tests/tail-2/inotify-race.sh: Likewise.
* tests/tail-2/inotify-rotate-resources.sh: Likewise.
* tests/tail-2/inotify-rotate.sh: Likewise.
* tests/tail-2/pid.sh: Likewise.
* tests/tail-2/pipe-f2.sh: Likewise.
* tests/tail-2/retry.sh: Likewise.
* tests/tail-2/symlink.sh: Likewise.
* tests/tail-2/tail-n0f.sh: Likewise.
* tests/tail-2/wait.sh: Likewise.
* tests/cp/existing-perm-race.sh: Likewise.
* tests/cp/file-perm-race.sh: Likewise.
* tests/cp/parent-perm-race.sh: Likewise.
* tests/cp/sparse-to-pipe.sh: Likewise.
* tests/dd/stats.sh: Likewise.
* tests/du/move-dir-while-traversing.sh: Likewise.
* tests/misc/cat-buf.sh: Likewise.
* tests/misc/help-version.sh: Likewise.
* tests/misc/printf-surprise.sh: Likewise.
* tests/misc/sort-compress-proc.sh: Likewise.
* tests/misc/sort-spinlock-abuse.sh: Likewise.
* tests/misc/stdbuf.sh: Likewise.
* tests/misc/tac-continue.sh: Likewise.
* tests/misc/timeout-group.sh: Likewise.
* tests/mv/i-3.sh: Likewise.
* tests/rm/dangling-symlink.sh: Likewise.
* tests/rm/isatty.sh: Likewise.
* cfg.mk (sc_prohibit_test_background_without_cleanup_):
A new syntax-check to ensure cleanup_() is defined
when background tasks are created in a test.

tests/du/move-dir-while-traversing.sh

@@ -71,10 +71,13 @@ for i in $(seq 50); do
   mkdir -p $t/3/a/b/c/$i/$long || framework_failure_
 done
 
+# Terminate any background cp process
+cleanup_() { kill $pid 2>/dev/null && wait $pid; }
+
 # Prohibit suspension, which could otherwise cause a timeout-induced FP failure.
 trap '' TSTP
 
-timeout 6 ./inotify-watch-for-dir-access.py $t/3/a/b > start-msg &
+timeout 6 ./inotify-watch-for-dir-access.py $t/3/a/b > start-msg & pid=$!
 
 # Wait for the watcher to start...
 nonempty() { test -s start-msg || { sleep $1; return 1; }; }