feat(Kokoro): add sub-directory support + split out lint build

.kokoro/lint.cfg

@@ -0,0 +1,12 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+    key: "TRAMPOLINE_IMAGE"
+    value: "gcr.io/cloud-devrel-kokoro-resources/php74"
+}
+
+env_vars: {
+    key: "RUN_CS_CHECK"
+    value: "true"
+}

.kokoro/lint.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+# Copyright 2021 Google Inc.
+#
+# 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.
+
+set -e
+
+if [ "${BASH_DEBUG}" = "true" ]; then
+    set -x
+fi
+
+# Kokoro directory for running these samples
+cd github/php-docs-samples
+
+mkdir -p build/logs
+
+export PULL_REQUEST_NUMBER=$KOKORO_GITHUB_PULL_REQUEST_NUMBER
+
+# Run code standards check when appropriate
+if [ "${RUN_CS_CHECK}" = "true" ]; then
+  bash testing/run_cs_check.sh
+fi

.kokoro/system_tests.sh

@@ -64,5 +64,11 @@ fi
 # Install global test dependencies
 composer install -d testing/
 
+# cd into specific subdirectory (if appropriate)
+MAIN_DIR=$(pwd)
+if [ ! -z "${SUB_DIRECTORY}" ]; then
+  cd $SUB_DIRECTORY
+fi
+
 # Run tests
-bash testing/run_test_suite.sh
+bash $MAIN_DIR/testing/run_test_suite.sh