Add test to assert unique ids in singlehtml output

Since the singlehtml aggregates all doc files into a single html page
during the write step, and the ids must be unique for proper link
anchoring, add test that collects all ids in the page and checks if all
ids are unique, by asserting the length of the list against it as a set.

Author: gastmaier

tests/test_builders/test_build_html_tocdepth.py

@@ -134,3 +134,16 @@ def test_tocdepth_singlehtml(
 ) -> None:
     app.build()
     check_xpath(cached_etree_parse(app.outdir / 'index.html'), 'index.html', *expect)
+
+@pytest.mark.sphinx('singlehtml', testroot='tocdepth')
+@pytest.mark.test_params(shared_result='test_build_html_tocdepth')
+def test_unique_ids_singlehtml(
+    app: SphinxTestApp,
+    cached_etree_parse: Callable[[Path], ElementTree],
+) -> None:
+    app.build()
+    tree = cached_etree_parse(app.outdir / 'index.html')
+    root = tree.getroot()
+
+    ids = [el.attrib['id'] for el in root.findall(".//*[@id]")]
+    assert len(ids) == len(set(ids))