Stabilize the numfig warning test

[DavidAGInnovation]

Summary

• stop sharing the numfig warning test's build result with the other HTML builder tests
• keep the test isolated so warning assertions do not depend on shared test output

Checks

• pytest -q tests/test_builders/test_build_html_numfig.py
• Ruff check and format validation for the changed file

Fixes #14104

[jdillard]

This follows up on #14100, which intended to isolate test_numfig_disabled_warn but changed the wrong tests and was subsequently reverted in #14106.

It also fixes #14104 and addresses the numfig flakiness mentioned in #13996, but doesn't close that broader issue.

Fix analysis

test_numfig_disabled_warn only checks that a build emits the “numfig is disabled” warning. It was marked with shared_result='test_build_html_numfig', same as the nearby HTML output tests, so they reuse one build for speed.

Unfortunately, the sharing is wrong for a warning assertion. With shared_result, if another test already built (or artifacts exist while the in-memory restore is empty), the app can skip rebuilding. Then app.warning stays empty and the assert fails intermittently under parallel CI, depending on which shared-result test runs the build first.

FAILED tests/test_builders/test_build_html_numfig.py::test_numfig_disabled_warn - AssertionError: assert 'index.rst:47: WARNING: numfig is disabled. :numref: is ignored.' in ''

This PR drops shared_result from that one test so it always does a fresh build and owns its warning stream. The other numfig tests can keep sharing; they only check HTML, not warnings.

[jdillard]

@picnixz what do you think of this fix?

The test_numfig_disabled_warn flaked on the Docutils HEAD job in #14574 after I merged in the two recent CI fixes. Seems like this PR would help to make CI even greener.