Make tests for URL load check rule functions self contained

[per1234]

Several rules check whether URLs provided in project metadata load.

Previously, the handling of various HTTP response status codes of these rule functions was tested by using the convenient httpstat.us site. Unfortunately the maintainers of that site shut it down months ago due to abuse. This caused the spurious failure of the unit tests. Although the httpstat.us maintainers state that they plan to eventually resume service, there hasn't been any indication of progress on that front.

So it will be best to make the tests self contained, without any dependence on an external site. This is accomplished by creating a local HTTP server that responds with the intended status code. Unfortunately this does require the addition of code to the test in order to generate test data on the fly in order to reference the arbitrary port of the test server.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.33%. Comparing base (c142981) to head (2a31091).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #963   +/-   ##
=======================================
  Coverage   83.33%   83.33%           
=======================================
  Files           1        1           
  Lines         180      180           
=======================================
  Hits          150      150           
  Misses         19       19           
  Partials       11       11           

Flag	Coverage Δ
unit	83.33% <ø> (ø)

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