tests: skip snipptes/samples in main Kokoro build

Toward #191.

Author: tseaver

.kokoro/build.sh

@@ -29,11 +29,13 @@ export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/service-account.json
 # Setup project id.
 export PROJECT_ID=$(cat "${KOKORO_GFILE_DIR}/project-id.json")
 
+export SESSIONS=$(nox --list | grep "^\*" | cut -d " " -f 2 | grep -v snippets)
+
 # Remove old nox
 python3.6 -m pip uninstall --yes --quiet nox-automation
 
 # Install nox
 python3.6 -m pip install --upgrade --quiet nox
 python3.6 -m nox --version
 
-python3.6 -m nox
+python3.6 -m nox $SESSIONS