tests: cleanup background processes upon interruption

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.