kokoro: add xds job (#3448)

test/kokoro/README.md

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+The scripts in this directory are intended to be run by Kokoro CI jobs.

test/kokoro/xds.cfg

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+# Config file for internal CI
+
+# Location of the continuous shell script in repository.
+build_file: "grpc-go/test/kokoro/xds.sh"
+timeout_mins: 90

test/kokoro/xds.sh

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+#!/bin/bash
+
+set -exu -o pipefail
+[[ -f /VERSION ]] && cat /VERSION
+
+cd github
+
+export GOPATH="${HOME}/gopath"
+pushd grpc-go/interop/xds/client
+go build
+popd
+
+git clone https://github.com/grpc/grpc.git
+
+grpc/tools/run_tests/helper_scripts/prep_xds.sh
+GRPC_GO_LOG_VERBOSITY_LEVEL=99 GRPC_GO_LOG_SEVERITY_LEVEL=info \
+  python3 grpc/tools/run_tests/run_xds_tests.py \
+    --test_case=all \
+    --project_id=grpc-testing \
+    --gcp_suffix=$(date '+%s') \
+    --verbose \
+    --client_cmd="grpc-go/interop/xds/client/client \
+      --server=xds-experimental:///{server_uri} \
+      --stats_port={stats_port} \
+      --qps={qps}"