I just discovered that Piglit now supports a "make install" target, so there is no need to build and run Piglit from its source/build tree: we can distribute it.
There is no released source archive. Upstream sources are available in freedesktop's Git repository. I have a mirror on my GitHub with a patch for FreeBSD in the master-freebsd branch (I still need to upstream it). The port uses USE_GITHUB to fetch sources from this branch.
All tests, comprised of native binaries, Python scripts and data files, are all installed in $LOCALBASE/lib. I didn't try to split that in lib/libdata/libexec/share as it would require patching Piglit and I see no good reason for it. The plist is huge with more than 28000 files.