Tests: add reference URI test coverage for 'singlehtml' builder

tests/roots/test-refuris/ch1.rst

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+Chapter One
+===========
+
+This chapter discusses :term:`API` design and :term:`REST` principles.
+
+We use :term:`API` throughout our application to communicate between
+different services.
+
+Section 1.1
+-----------
+
+More details about :term:`REST` architecture here.
+
+The :term:`API` should follow RESTful conventions.

tests/roots/test-refuris/conf.py

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+project = 'Glossary Test'
+extensions = []

tests/roots/test-refuris/glossary/term1.rst

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+.. glossary::
+
+   API
+      Lorem Ipsum.

tests/roots/test-refuris/glossary/term2.rst

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+.. glossary::
+
+   API
+      Lorem Ipsum.

tests/roots/test-refuris/index.rst

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+Main Documentation
+==================
+
+Welcome to the documentation.
+
+.. toctree::
+   :maxdepth: 2
+
+   ch1
+   glossary/term1
+   glossary/term2

tests/test_builders/test_build_html_refuris.py

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+"""Test output of reference URIs when building single-page HTML output."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+from tests.test_builders.xpath_util import check_xpath
+
+if TYPE_CHECKING:
+    from collections.abc import Callable, Sequence
+    from pathlib import Path
+    from xml.etree.ElementTree import Element, ElementTree
+
+    from sphinx.testing.util import SphinxTestApp
+
+
+def _internal_reference_fragment_check(nodes: Sequence[Element]) -> None:
+    """Confirm that internal references do not contain duplicate fragment symbols"""
+    assert nodes, 'Expected at least one node to check'
+    for node in nodes:
+        assert node.tag == 'a', 'Attempted to check hyperlink on a non-anchor element'
+        href = node.attrib.get('href')
+        # Allow Sphinx index and table hyperlinks to be non-same-document, as exceptions.
+        if href in {'genindex.html', 'py-modindex.html', 'search.html'}:
+            continue
+        assert not href or href.count('#') < 2, 'Hyperlink contains duplicate fragments'
+
+
+@pytest.mark.sphinx('singlehtml', testroot='refuris')
+def test_singlehtml_refuris(
+    app: SphinxTestApp,
+    cached_etree_parse: Callable[[Path], ElementTree],
+) -> None:
+    app.build()
+    check_xpath(
+        cached_etree_parse(app.outdir / 'index.html'),
+        'index.html',
+        ".//a[@class='reference internal']",
+        _internal_reference_fragment_check,
+    )