HTML builder: toctree reliability during parallel builds

[jayaddison]

Feature or Bugfix

• Bugfix

Purpose

• This changeset is intended to resolve a toctree output nondeterminism problem that I believe may be the cause of both sphinx-build -jauto has nondeterministic html results #6714 and BUG: env.toctree_includes different between parallel and serial builds #12409.
• A fix is pending -- to begin with this pull request offers test coverage that should expose the problem (albeit intermittently, because some parallelised builds do produce correct output).

Detail

• Extend the test coverage on the toctree-glob testroot to enable a parallelised test where the alabaster theme is used (required so that a navigation sidebar, where the problem manifests, is included in the output)

Relates

• May resolve sphinx-build -jauto has nondeterministic html results #6714.
• May resolve BUG: env.toctree_includes different between parallel and serial builds #12409.

[jayaddison]

This test case does pass intermittently for me locally, and that's what I was hoping to demonstrate with a few small commits here. It seems I may be wasting CI compute resources though; the test always fails when run on a host with a single core available, so.. perhaps these are always going to fail.

[jayaddison]

I'll push one more commit, and then I'll give up on demonstrating the intermittent nature of this using CI and will move on to investigating possible fixes.

[jayaddison]

Confusingly to me, the test_parallel_toctree test case added here now fails reliably on my multi-core local machine. It was previously failing intermittently in a way that I believe confirmed something about the non-deterministic toctree construction behaviour.

Because this changeset now seems invalid (it doesn't prove any nondeterministic behaviour) I'm going to close this pull request.