Add test with two kinds of file creation orders

Add test with two kinds of file creation orders

Data loss was identified in #7401 when many small files were copied.
This adds a reproducer for this bug and other similar ones: randomly
generate N files. Then, listing M of them by `ls -U` order, produce
those same files in a directory of the same name.

This triggers the bug consistently, provided N and M are large enough.
Here, N=2^16 and M=2^13.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #7411