Don't use the .so extension on test assets

Don't use the .so extension on test assets

Some packaging systems, such as dpkg-source in native mode used by
Debian's daily build system, will filter out files considered unclean
prior to the build. This behavior causes our test assets to be deleted
which, subsequently, fails the make check.

Since we do not require the .so extension and these files should be
treated as blobs, we rename them to "_so".

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>