Fail tests making external HTTP requests that are not in the `external-http` group.

[peterwilsoncc]

While reviewing the patch for #62325, I decided it would be helpful to have two tests:

• Ensure that any test making an external HTTP request was in the group external-http
• Ensure that any test in the external-http group did, in fact, make a request

I did this in the hackiest way possible (see the attachment) for the purposes of getting the review done but I am wondering if it's worth adding such tests to the test suite to ensure that HTTP connection errors don't affect the main test suite.

I'm not sure if it's a good or bad idea but would appreciate others weighing in with their views.