Create helper function(s) to create simulated TPMs with EK certificates.

Currently the simulated TPM doesn't have any EK certificates. This makes it hard to test the [tpm.ekcertificates()](https://github.com/google/go-attestation/blob/master/attest/tpm.go#L408C2-L408C16) method. This lack of testing also has come up in #468 