MuleSoft - mock httpClient.send for extension unit tests

Question

I have created a custom extension (Connector), which sends an HttpRequest (using org.mule.runtime.http.api.client.HttpClient and the related classes).

The extension's unit tests file contains the following test, to which I've added a simple Mockito mock to throw a TimeoutException when the HTTP request is being sent:

public class DemoOperationsTestCase extends MuleArtifactFunctionalTestCase {

  /**
   * Specifies the mule config xml with the flows that are going to be executed in the tests, this file lives in the test resources.
   */
  @Override
  protected String getConfigFile() {
    return "test-mule-config.xml";
  }

  @Test
  public void executeSayHiOperation() throws Exception {
      HttpClient httpClient = mock(HttpClient.class);
      HttpRequest httpRequest = mock(HttpRequest.class);
      when(httpClient.send(any(HttpRequest.class), anyInt(), anyBoolean(), any(HttpAuthentication.class))).thenThrow(new TimeoutException());
      String payloadValue = ((String) flowRunner("sayHiFlow").run()
                                      .getMessage()
                                      .getPayload()
                                      .getValue());
      assertThat(payloadValue, is("Hello Mariano Gonzalez!!!"));
  }
}

The test should fail because the function should throw a TimeoutException, it is what I want for now.

The code that is being tested is as follows (redacted for convenience):

HttpClient client = connection.getHttpClient();
HttpResponse httpResponse = null;
String response = "N/A";
HttpRequestBuilder builder = HttpRequest.builder();

try {
    httpResponse = client
            .send(builder
                    .addHeader("Authorization", authorization)
                    .method("POST")
                    .entity(new ByteArrayHttpEntity("Hello from Mule Connector!".getBytes()))
                    .uri(destinationUrl)
                    .build(),
                    0, false, null);
    response = IOUtils.toString(httpResponse.getEntity().getContent());
} catch (IOException e) {
    e.printStackTrace();
    throw new ModuleException(DemoError.NO_RESPONSE, new Exception("Failed to get response"));
} catch (TimeoutException e) {
    e.printStackTrace();
    throw new ModuleException(DemoError.NO_RESPONSE, new Exception("Connection timed out"));
}

But I always get the "Failed to get response" error message, which is what I get when I run the Connector with a nonexistent server, therefore the mock isn't working (it actually tries to send an HTTP request).

I am new to Java unit testing, so it might be a general mocking issue and not specific to MuleSoft - though I came across other questions (such as this one and this one), I tried the suggestions in the answers and the comments, but I get the same error. I even tried to use thenReturn instead of thenThrow, and I get the same error - so the mock isn't working.

Any idea why this is happening?