cp: Adjust the sparse file tests.

* The sparsity check was ineffective: it compared the apparent size in bytes to the actual size in blocks. Instead, write a tool that reliably detects sparseness.
* Some of the seq commands were missing an argument.
* Based on empirical evidence, 1 MB holes are not necessarily large enough to be preserved by the underlying filesystem. Increase the hole size to 16 MB.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: cracauer
Differential Revision: https://reviews.freebsd.org/D38414