`IDTokenVerifierTest` suite should be adapted for a Kokoro integration test

[diegomarquezp]

This test class uses hardcoded tokens that need to be carefully curated before being exposed in the repo (as in this recent PR), implying a security risk. We should consider obtaining tokens programatically and passing them to this test in order to not expose them, probably from a Kokoro environment.

CC: @blakeli0