Add sanity check preventing "DO NOT SUBMIT"s from submitting (#27250)

tools/run_tests/sanity/check_do_not_submit.sh

@@ -0,0 +1,24 @@
+#! /bin/bash
+# Copyright 2021 The gRPC Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# Checks if any file contains "DO NOT SUBMIT"
+
+cd "$(dirname "$0")/../../.." || exit 1
+grep -Irn \
+  --exclude='check_do_not_submit.sh' \
+  --exclude-dir='.git/' \
+  --exclude-dir='third_party/' \
+  'DO NOT SUBMIT'
+test $? -eq 1 || exit 1

tools/run_tests/sanity/sanity_tests.yaml

@@ -4,6 +4,7 @@
 - script: tools/run_tests/sanity/check_buildifier.sh
 - script: tools/run_tests/sanity/check_cache_mk.sh
 - script: tools/run_tests/sanity/check_deprecated_grpc++.py
+- script: tools/run_tests/sanity/check_do_not_submit.sh
 - script: tools/run_tests/sanity/check_illegal_terms.sh
 - script: tools/run_tests/sanity/check_owners.sh
 - script: tools/run_tests/sanity/check_port_platform.py