Follow the instructions to create a root certificate, then trust it and create a child certificate from it.

Since the root certificate of client_cert.pfx is obviously not trusted automatically by cloning this repo, the tests in WireMockServerTests.ClientCertificate.cs set WireMockServerSettings.AcceptAnyClientCertificate to true so that tests pass even if the device hasn't trusted the root of client_cert.pfx.