[tests] linkcheck: expect and check for a docutils raw HTTP request during testsuite.

[jayaddison]

Feature or Bugfix

• Bugfix (improved test isolation)

Purpose

• Improve test isolation of the linkcheck builder test_raw_node test case.

Detail

• Patch a docutils HTTP request that occurs when processing a raw directive during testing.

Relates

• Discovered during investigation of [tests] linkcheck builder unit tests fail sporadically when run using unpredictable orderings. #12122.

[jayaddison]

Apologies for the notification noise from opening this pull request; I've found a better approach that doesn't require mocking and that is compatible across the versions of docutils that we test with.