add support for clean test output (#29)

Author: jdnurme

kokoro/presubmit.cfg

@@ -13,3 +13,9 @@
 # limitations under the License.
 
 build_file: "dataflux-client-python/kokoro/build.sh"
+
+action {
+  define_artifacts {
+    regex: "**/unit_tests/sponge_log.xml"
+  }
+}