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

Start with tpm.ekcertificates() in attest/tpm.go and inspect the simulated TPM support, then review issue #468 for related testing context. Add helper function(s) that create simulated TPMs with EK certificates and cover the missing certificate behavior with tests; done when ekcertificates() can be tested against those simulated TPMs.

Written by the indexing model from the issue text.