[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.

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

    var builder: HeldCertificate.Builder = HeldCertificate.Builder()
    builder = builder.validityInterval(0L, 0L)
    builder = builder.duration(0L, TimeUnit.SECONDS)
    builder = builder.addSubjectAlternativeName("")
    builder = builder.commonName("")
    builder = builder.organizationalUnit("")
    builder = builder.serialNumber(BigInteger.ZERO)
    builder = builder.serialNumber(0L)