[Services] Fix tunnelbroker ctest usage

Summary:
Update gtest usage to be more canonical

Depends on D4781

Test Plan:

nix develop

cd services/tunnelbroker
export COMM_TEST_SERVICES=1
rm -rf build
mkdir build
cd build

cmake ..
make -j
ctest # test suite should run

If you want the test suite to fully pass, you need to also follow
https://github.com/CommE2E/comm/blob/master/docs/dev_services.md#rabbitmq-tunnelbroker-only

Unfortunately, the logic for finding the tunnelbroker.ini file still
assumes docker-only paths

Reviewers: atul, max, karol

Reviewed By: atul, max

Subscribers: ashoat, tomek, adrian, abosh

Differential Revision: https://phab.comm.dev/D4782