copy: Skip some tests when running on ZFS

ZFS doesn't work as we expect when trying to create a preallocated file, and this breaks a couple of nbdcopy tests that try to test --allocated + --destination-is-zero flags together. Add a new test that checks that sparse and preallocated files work as we expect on the current filesystem, and skip these tests according to the outcome.

NB: If testing this locally, you will need to do this after applying the patch:

make -C tests functions.sh