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.

tests/dd/stats.sh

@@ -67,7 +67,7 @@ done
 
 progress_output()
 {
-  { sleep "$1"; echo 1; } | dd bs=1 status=progress of=/dev/null 2>err
+  { sleep $1; echo 1; } | dd bs=1 status=progress of=/dev/null 2>err
   # Progress output should be for "byte copied", while final is "bytes ..."
   grep 'byte copied' err
 }