Thanks, this clears up the mystery! -- I made the mistake of choosing
the eCryptfs-homedir option when installing Ubuntu, so now I have eCryptfs on top of LVM on top of full-disk encryption.

So I can't create sparse files in my home directory, but I *can*
create sparse files with zero overhead in /tmp.


The test suite runs smoothly from /usr/local.  Should we perhaps create
temp files in /tmp rather than in a subdir of the build directory?