Adjust sparse restore tests for test fs block size

Adjust sparse restore tests for test fs block size

Change test-sparse-files.sh to detect the test fs block size (when
possible) and adjust its behavior accordingly.  If the block size can't
be determined, use a block size of 3MB, which is hoped to be larger than
any block sizes we'll encounter anytime soon.

Previously the tests might fail on filesystems with relatively large
block sizes, like those on the current Debian powerpc and ppc64el build
daemons (64k).

Thanks to Goswin Brederlow for mentioning that the Lucene block size is
1MB, to Robert Edmonds for running a build through the Debian buildds,
which revealed the problem, and to Julien Cristau for reporting the
block size on the failing buildds.

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