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 <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Antonio Russo <[email protected]>
Closes #7411 

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