Test sparse restore of short in-buffer zero run

Test sparse restore of short in-buffer zero run

Test that sparse --restore handles the case where within one call to
write_sparsely() (one buffer) we have non-zero bytes, followed by a run
of zero bytes that's longer than the minimum sparse run
length (currently 512), followed by non-zero bytes.

Currently, the initial non-zero bytes will be lost, and this test will
fail.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>