crypto/tls: Unit tests for ECH edge caes

[cjpatton]

@Lekensteyn suggested we add a unit test for asserting that the client-facing server rejects if the client sends GREASE ECH with a config id that matches a known config but which uses a different KEM algorithm: https://github.com/cloudflare/go/pull/95#issuecomment-882700600

We might also want to check that it rejects when the KDF/AEAD algorithm pair isn't supported.