make test: support out-of-tree plugins

usage:

env EXTERN_PLUGINS=/path/to/plugins make test

Change-Id: I8eece726dfafeff1cffd921c1e18cd3eb7eb64ed
Signed-off-by: Klement Sekera <ksekera@cisco.com>
diff --git a/test/Makefile b/test/Makefile
index c65eaae..6647d67 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -92,6 +92,8 @@
 		echo VPP_TEST_BIN=$(VPP_TEST_BIN);\
 		echo VPP_TEST_PLUGIN_PATH=$(VPP_TEST_PLUGIN_PATH);\
 		echo VPP_TEST_INSTALL_PATH=$(VPP_TEST_INSTALL_PATH);\
+		echo EXTERN_TESTS=$(EXTERN_TESTS);\
+		echo EXTERN_PLUGINS=$(EXTERN_PLUGINS);\
 		echo LD_LIBRARY_PATH=$(LD_LIBRARY_PATH);\
 		echo '***';\
 		exec </dev/tty" | bash -i
@@ -182,6 +184,8 @@
 	@echo " COREDUMP_SIZE=<size> - pass <size> as unix { coredump-size <size> } argument to vpp"
 	@echo "                        e.g. COREDUMP_SIZE=4g"
 	@echo "                             COREDUMP_SIZE=unlimited"
+	@echo " EXTERN_TESTS=<path>  - path to out-of-tree test_<name>.py files containing test cases"
+	@echo " EXTERN_PLUGINS=<path>- path to out-of-tree plugins to be loaded by vpp under test"
 	@echo ""
 	@echo "Creating test documentation"
 	@echo " test-doc            - generate documentation for test framework"