`IdTokenVerifierTest` needs to be stable

[diegomarquezp]

So far we use a SA token that will not be valid once the certificates rotate. Capture a certificate and use it to intercept the verification process to ensure stability.

[diegomarquezp]

The tests use a fixed clock and should not have certificate expiration problems.

[diegomarquezp]

See also: #1055