build: run browserstack on circleci (#13749)

scripts/circleci/run-browserstack-tests.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+# In case any command failed, we want to immediately exit the script with the
+# proper exit code.
+set -e
+
+# Go to project directory.
+cd $(dirname ${0})/../..
+
+# Decode access token and make it accessible for child processes.
+export BROWSER_STACK_ACCESS_KEY=`echo ${BROWSER_STACK_ACCESS_KEY} | rev`
+
+# Start tunnel and wait for it being ready.
+./scripts/browserstack/start-tunnel.sh
+./scripts/browserstack/wait-tunnel.sh
+
+# Setup the test platform environment variable that will be read
+# by the Karma configuration script.
+export TEST_PLATFORM="browserstack"
+
+# Run the unit tests on Browserstack with Karma.
+yarn gulp ci:test
+
+# Kill the Browserstack tunnel. This is necessary in order to avoid rate-limit
+# errors that cause the unit tests to be flaky.
+./scripts/browserstack/stop-tunnel.sh