Add test filtering to Kokoro PR jobs

tools/internal_ci/helper_scripts/prepare_build_linux_rc

@@ -31,4 +31,11 @@ PYTHONWARNINGS=ignore XDG_CACHE_HOME=/tmp/xdg-cache-home sudo -E pip install cov
 # Download Docker images from DockerHub
 export DOCKERHUB_ORGANIZATION=grpctesting
 
+# If this is a PR using RUN_TESTS_FLAGS var, then add flags to filter tests
+if [ -n "$KOKORO_GITHUB_PULL_REQUEST_NUMBER" ] && [ -n "$RUN_TESTS_FLAGS" ]; then
+  sudo apt-get install -y jq
+  ghprbTargetBranch=$(curl -s https://api.github.com/repos/grpc/grpc/pulls/$KOKORO_GITHUB_PULL_REQUEST_NUMBER | jq -r .base.ref)
+  export RUN_TESTS_FLAGS="$RUN_TESTS_FLAGS --filter_pr_tests --base_branch origin/$ghprbTargetBranch"	
+fi
+
 git submodule update --init

tools/internal_ci/linux/sanitizer/pull_request/grpc_c_asan.cfg

@@ -27,5 +27,5 @@ action {
 
 env_vars {
   key: "RUN_TESTS_FLAGS"
-  value: "-f c asan --inner_jobs 16 -j 1 --internal_ci --filter_pr_tests --base_branch origin/master --max_time=3600"
+  value: "-f c asan --inner_jobs 16 -j 1 --internal_ci --max_time=3600"
 }

tools/internal_ci/linux/sanitizer/pull_request/grpc_c_msan.cfg

@@ -27,5 +27,5 @@ action {
 
 env_vars {
   key: "RUN_TESTS_FLAGS"
-  value: "-f c msan --inner_jobs 16 -j 1 --internal_ci --filter_pr_tests --base_branch origin/master --max_time=3600"
+  value: "-f c msan --inner_jobs 16 -j 1 --internal_ci --max_time=3600"
 }

tools/internal_ci/linux/sanitizer/pull_request/grpc_c_tsan.cfg

@@ -27,5 +27,5 @@ action {
 
 env_vars {
   key: "RUN_TESTS_FLAGS"
-  value: "-f c tsan --inner_jobs 16 -j 1 --internal_ci --filter_pr_tests --base_branch origin/master --max_time=3600"
+  value: "-f c tsan --inner_jobs 16 -j 1 --internal_ci --max_time=3600"
 }

tools/internal_ci/linux/sanitizer/pull_request/grpc_c_ubsan.cfg

@@ -27,5 +27,5 @@ action {
 
 env_vars {
   key: "RUN_TESTS_FLAGS"
-  value: "-f c ubsan --inner_jobs 16 -j 1 --internal_ci --filter_pr_tests --base_branch origin/master --max_time=3600"
+  value: "-f c ubsan --inner_jobs 16 -j 1 --internal_ci --max_time=3600"
 }

tools/internal_ci/linux/sanitizer/pull_request/grpc_cpp_asan.cfg

@@ -27,5 +27,5 @@ action {
 
 env_vars {
   key: "RUN_TESTS_FLAGS"
-  value: "-f c++ asan --inner_jobs 16 -j 1 --internal_ci --filter_pr_tests --base_branch origin/master --max_time=3600"
+  value: "-f c++ asan --inner_jobs 16 -j 1 --internal_ci --max_time=3600"
 }

tools/internal_ci/linux/sanitizer/pull_request/grpc_cpp_tsan.cfg

@@ -27,5 +27,5 @@ action {
 
 env_vars {
   key: "RUN_TESTS_FLAGS"
-  value: "-f c++ tsan --inner_jobs 16 -j 1 --internal_ci --filter_pr_tests --base_branch origin/master --max_time=3600"
+  value: "-f c++ tsan --inner_jobs 16 -j 1 --internal_ci --max_time=3600"
 }