ci: move local and saucelabs unit tests to circle (#27937)

Moving the tests over to CircleCI in pretty much "as-is" state just so that we can drop the dependency on Travis.

In the followup changes we plan to migrate these tests to run on sauce under bazel. @gregmagolan is working on that.

I've previously verified that all the tests executed in legacy-unit-tests-local already under bazel.
Therefore the legacy-unit-tests-local job is strictly not necessary any more, but given how flaky legacy-unit-tests-saucelabs is,
it is good to have the -local job just so that we can quickly determine if any failure is a flake or legit issue
(the bazel version of these tests could theoretically run in a slightly different way and fail or not fail in a different way, so having -lcoal job is just an extra safety check).

This change was coauthored with @devversion

PR Close #27937

scripts/saucelabs/stop-tunnel.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+# Disable printing of any executed command because this would cause a lot
+# of spam due to the loop.
+set +x -u -e -o pipefail
+
+if [[ ! -f ${SAUCE_PID_FILE} ]]; then
+  echo "Could not find Saucelabs tunnel PID file. Cannot stop tunnel.."
+  exit 1
+fi
+
+echo "Shutting down Sauce Connect tunnel"
+
+# The process id for the sauce-connect instance is stored inside of the pidfile.
+tunnelProcessId=$(cat ${SAUCE_PID_FILE})
+
+# Kill the process by using the PID that has been read from the pidfile. Note that
+# we cannot use killall because CircleCI base container images don't have it installed.
+kill ${tunnelProcessId}
+
+while (ps -p ${tunnelProcessId} &> /dev/null); do
+  printf "."
+  sleep .5
+done
+
+echo ""
+echo "Sauce Connect tunnel has been shut down"