SEBA-902 single-olt tests;
Pin protoc-gen-go to 1.3.2 to resolve compatibility issue;
Run go mod tidy / go mod vendor on importer;
Add Go Module support to demotest

Change-Id: Ifde824fc9a6317b0adc1e12bea54ee1f9b788906
diff --git a/demo_test/vendor/google.golang.org/grpc/AUTHORS b/demo_test/vendor/google.golang.org/grpc/AUTHORS
new file mode 100644
index 0000000..e491a9e
--- /dev/null
+++ b/demo_test/vendor/google.golang.org/grpc/AUTHORS
@@ -0,0 +1 @@
+Google Inc.