[selectmenu] Exclude selectmenu tests on LayoutNG disabled builder

After crrev.com/c/4082046, the feature now indirectly depends on
LayoutNG via CSS Anchor Position. So it no longer makes sense to
test it when LayoutNG is disabled.

Change-Id: I37bab659d962c44ff5c7514c514c9dfdf54d801e
Fixed: 1399750
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4099015
Reviewed-by: Mason Freed <masonf@chromium.org>
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1082665}