Show module file path in TF_HEADER

tests/TF_HEADER

 
 echo "version: $(saturnin-demo -V)"
 echo "binary: $(which saturnin-demo)"
+echo "SHELLFU_PATH: ${SHELLFU_PATH:-null}"
+echo "shellfu-modules:"
+echo "    saturnin_demo_greet: $(shellfu _select_mfile saturnin_demo_greet)"
 echo "saturnin-version: $(saturnin-demo --saturnin-get-saturnin-version)"