Dump some triples for a failing materializer test.

This change augments the first materializer test to dump out
some triples on test failure, to assist with debugging.

make prepush passes.

no release note needed, change to test code.

Change-Id: I1495aaf87bcfdf42313d73ae5663171943bd3d46
Reviewed-on: https://gerrit.franz.com:9080/2740
Reviewed-by: Ahmon Dancy <dancy@franz.com>
Tested-by: Kevin Layer <layer@franz.com>