[FIX] web: <details> visiblity check regression in Chromium 97+

Since Chromium 97, folded `<details>` content has non-zero computed
width and height which returns false-positive QUnit
`assert.isNotVisible()` calls.

In a nutshell, this was introduced in https://bugs.chromium.org/p/chromium/issues/detail?id=1185950
when Chromium implemented the ability for "Find in page" to search in folded
`<details>`. To do so, the `content-visibility` rule was used instead of
`display: none` which, even if functionally similar, has an impact on the
values returned by `getBoundingClientRect()` (i.e. used in
`assert.isNotVisible()`.

A regression report was filled in https://bugs.chromium.org/p/chromium/issues/detail?id=1276028
to highlight this change of behavior. What emerged from this report's
discussion is that Chromium has no intent to revert this change and
instead put the emphasis on avoiding to use `getBoundingClientRect()`
and similar methods for visibility check (even pushing a proposal for a
`Element.isVisible()` API; cf. w3c/csswg-drafts#6850).

This commit works around this issue by forcing the content to `display:
none`, like in the previous Chromium versions.

X-original-commit: 5aa2a71

addons/web/static/tests/qunit.js

@@ -409,4 +409,16 @@
             addSortButton();
         }
     });
+
+    // -----------------------------------------------------------------------------
+    // FIXME: This sounds stupid, it feels stupid... but it fixes visibility check in folded <details> since Chromium 97+ 💩
+    // Since https://bugs.chromium.org/p/chromium/issues/detail?id=1185950
+    // See regression report https://bugs.chromium.org/p/chromium/issues/detail?id=1276028
+    // -----------------------------------------------------------------------------
+
+    QUnit.begin(() => {
+        const el = document.createElement("style");
+        el.innerText = "details:not([open]) > :not(summary) { display: none; }";
+        document.head.appendChild(el);
+    });
 })();