static void assertSuccessful(Response response) throws IOException {
    assertThat(response.isSuccessful())
      .withFailMessage("unsuccessful %s: %s", response.request(),
        response.peekBody(Long.MAX_VALUE).string())
      .isTrue();
  }
This POSTs a single span. Afterwards, we expect this trace in storage, and also the self-trace
of POSTing it.
zipkin-server/src/test/java/zipkin2/server/internal/brave/ITZipkinSelfTracing.java