tests: adjust delays to avoid false positives

tests: adjust delays to avoid false positives

* tests/rm/dangling-symlink.sh: The effective delay was only
1.5s.  So delay before the operation, and increase the iteration
count by 1 to increase the delay to 6.3s.  This failure was
noticed once on a FreeBSD 10.2 x86_64 virtual machine.
* tests/dd/stats.sh: Remove quotes on sleep argument
to ensure arguments are accumulated appropriately.
* tests/du/move-dir-while-traversing.sh: sleep before operation,
to increase the effective delay from 1.6s to 3.2s.
* tests/tail-2/flush-initial.sh: Likewise.
* tests/tail-2/pipe-f2.sh: Sleep first so that the effective delay
matches the commented value.