restore: add generative --sparse testing

restore: add generative --sparse testing

Add t/sparse-test-data to automatically generate cases that might trip
up "restore --sparse", and use it in test-sparse-files.sh.  This change
causes the current --sparse code to fail if you run enough trials,
but not as quickly as the existing "bup random" based test.

As compared to the existing "bup random" test, this one should evaluate
larger sparse regions much more often.

Thanks to Frank Gevaerts for help evaluating the fix.

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