Use tar-pax format

GNU docs say the format is "new" but was standardized in 2001.

With the older tar-ustar format, we were failing "make distcheck"
thanks to a 100+-character regression test filename that would end up
omitted (silently!?!) from the dist tarballs.  We could rename our
long-named tests, but doxygen seems to like generating huge filenames
from huge class names and I don't want to risk future equally-subtle
bugs.