[builder][held_certificate_builder] to create a self-signed certificate that a test server can use
for HTTPS:

```java
HeldCertificate localhostCertificate = new HeldCertificate.Builder()
    .addSubjectAlternativeName("localhost")
    .build();
```

[`HandshakeCertificates`][handshake_certificates] keeps the certificates for a TLS handshake.

      HeldCertificate
        .Builder()
        .certificateAuthority(0)
        .build()
    val serverCertificate =
      HeldCertificate
        .Builder()
        .signedBy(serverCa)
        .addSubjectAlternativeName(server.hostName)
        .build()
    val serverHandshakeCertificates =
      HandshakeCertificates
        .Builder()
        .addTrustedCertificate(clientCa.certificate)

      HeldCertificate
        .Builder()
        .certificateAuthority(0)
        .build()
    val serverCertificate =
      HeldCertificate
        .Builder()
        .signedBy(serverCa)
        .addSubjectAlternativeName(server.hostName)
        .build()
    val serverHandshakeCertificates =
      HandshakeCertificates
        .Builder()
        .addTrustedCertificate(clientCa.certificate)

    server.enqueue(MockResponse())
    val serverCertificate =
      HeldCertificate
        .Builder()
        .commonName("localhost") // Unusued for hostname verification.
        .addSubjectAlternativeName("wronghostname")
        .build()
    val serverCertificates =
      HandshakeCertificates
        .Builder()
        .heldCertificate(serverCertificate)
        .build()