overlay/066: adjust test file size && add more test patterns

overlay/066: adjust test file size && add more test patterns

Making many small holes in 10M test file seems not very
helpful for test coverage and it takes too much time on
creating test files. In order to improve test speed we
adjust test file size to (10 * iosize) for iosize aligned
hole files and meanwhile add more test patterns for small
random holes and small empty file.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>