Convert sa_test.go to use test.ThrowawayCert

[aarongable]

We have test.ThrowawayCert() specifically to generate certs for use in tests. We use it extensively in precertificates_test.go, but not in sa_test.go, presumably because that code is older and predates the helper method.

We should update the tests in sa_test.go to use test.ThrowawayCert(), and delete the on-disk test certificates (e.g. "www.eff.org.der", "test-cert.der") once they're no longer used.