Update a few <selectmenu> tests

These were missed in [1].

[1] https://chromium-review.googlesource.com/c/chromium/src/+/3594385

Bug: 1307772
Change-Id: I5b54213b51ffa39fa71f78c4161e84ff96e2c1ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3597516
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Dan Clark <daniec@microsoft.com>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#994341}
diff --git a/html/semantics/forms/the-selectmenu-element/selectmenu-parts-structure.tentative.html b/html/semantics/forms/the-selectmenu-element/selectmenu-parts-structure.tentative.html
index 683ca5a..927b45c 100644
--- a/html/semantics/forms/the-selectmenu-element/selectmenu-parts-structure.tentative.html
+++ b/html/semantics/forms/the-selectmenu-element/selectmenu-parts-structure.tentative.html
@@ -312,7 +312,8 @@
     const option2 = document.getElementById("selectMenu9-option2");
     assert_equals(selectMenu.value, "one", "Initial value should be the first option");
 
-    let newListbox = document.createElement("popup");
+    let newListbox = document.createElement("div");
+    newListbox.setAttribute("popup", "popup");
     newListbox.setAttribute("behavior", "listbox");
     let newOption = document.createElement("option");
     newOption.innerText = "three";
@@ -400,7 +401,8 @@
     await clickOn(selectMenuOption1);
     assert_false(selectMenu.open);
 
-    let newListbox = document.createElement("popup");
+    let newListbox = document.createElement("div");
+    newListbox.setAttribute("popup", "popup");
     newListbox.setAttribute("behavior", "listbox");
     let newOption = document.createElement("option");
     newOption.innerText = "three";