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/.travis.yml b/demo_test/vendor/google.golang.org/grpc/.travis.yml
new file mode 100644
index 0000000..a11e8cb
--- /dev/null
+++ b/demo_test/vendor/google.golang.org/grpc/.travis.yml
@@ -0,0 +1,42 @@
+language: go
+
+matrix:
+  include:
+  - go: 1.13.x
+    env: VET=1 GO111MODULE=on
+  - go: 1.13.x
+    env: RACE=1 GO111MODULE=on
+  - go: 1.13.x
+    env: RUN386=1
+  - go: 1.13.x
+    env: GRPC_GO_RETRY=on
+  - go: 1.13.x
+    env: TESTEXTRAS=1
+  - go: 1.12.x
+    env: GO111MODULE=on
+  - go: 1.11.x
+    env: GO111MODULE=on
+  - go: 1.9.x
+    env: GAE=1
+
+go_import_path: google.golang.org/grpc
+
+before_install:
+  - if [[ "${GO111MODULE}" = "on" ]]; then mkdir "${HOME}/go"; export GOPATH="${HOME}/go"; fi
+  - if [[ -n "${RUN386}" ]]; then export GOARCH=386; fi
+  - if [[ "${TRAVIS_EVENT_TYPE}" = "cron" && -z "${RUN386}" ]]; then RACE=1; fi
+  - if [[ "${TRAVIS_EVENT_TYPE}" != "cron" ]]; then export VET_SKIP_PROTO=1; fi
+
+install:
+  - try3() { eval "$*" || eval "$*" || eval "$*"; }
+  - try3 'if [[ "${GO111MODULE}" = "on" ]]; then go mod download; else make testdeps; fi'
+  - if [[ -n "${GAE}" ]]; then source ./install_gae.sh; make testappenginedeps; fi
+  - if [[ -n "${VET}" ]]; then ./vet.sh -install; fi
+
+script:
+  - set -e
+  - if [[ -n "${TESTEXTRAS}" ]]; then examples/examples_test.sh; interop/interop_test.sh; make testsubmodule; exit 0; fi
+  - if [[ -n "${VET}" ]]; then ./vet.sh; fi
+  - if [[ -n "${GAE}" ]]; then make testappengine; exit 0; fi
+  - if [[ -n "${RACE}" ]]; then make testrace; exit 0; fi
+  - make test