sbin/bectl: Skip tests if sparse files are not supported

[arichardson]

Summary

The tests create a 1GB test file and this causes the tests to fail in the
CheriBSD CI setup where we run tests with a tmpfs mount on /tmp. Tmpfs
does not support sparse files and it appears that tmpfs default to creating
a 1GB mount, so there is not enough space to run these tests.

Instead of checking for at least 1GB of free space this commit skips the
tests on file systems that do not support sparse files.

[kevans]

I'm contemplating dropping the disk size to 200M or less, but that'll require a fair amount of testing since I suspect nobody does sub-1G disks with ZFS :-). This works, and I'll rip it out should I get around to fixing the underlying cause.