Fix CI: .validationMessage assertion

Resolve a [CI failure][] related to a hard-coded English validation
message expectation failing due to an internationalized actual
validation message value.

[CI failure]: https://github.com/basecamp/trix/actions/runs/11317691021/job/31471435810#step:5:75

src/test/system/custom_element_test.js

@@ -1,4 +1,4 @@
-import { rangesAreEqual } from "trix/core/helpers"
+import { makeElement, rangesAreEqual } from "trix/core/helpers"
 import TrixEditorElement from "trix/elements/trix_editor_element"
 
 import {
@@ -635,6 +635,7 @@ testGroup("form property references its <form>", { template: "editors_with_forms
   testIf(TrixEditorElement.formAssociated, "validates with [required] attribute as invalid", () => {
     const editor = document.getElementById("editor-with-ancestor-form")
     const form = editor.form
+    const invalidInput = makeElement("input", { required: true })
     let invalidEvent, submitEvent = null
 
     editor.addEventListener("invalid", event => invalidEvent = event, { once: true })
@@ -646,7 +647,7 @@ testGroup("form property references its <form>", { template: "editors_with_forms
     // assert.equal(document.activeElement, editor, "editor receives focus")
     assert.equal(editor.required, true, ".required property retrurns true")
     assert.equal(editor.validity.valid, false, "validity.valid is false")
-    assert.equal(editor.validationMessage, "Please fill out this field.", "sets .validationMessage")
+    assert.equal(editor.validationMessage, invalidInput.validationMessage, "sets .validationMessage")
     assert.equal(invalidEvent.target, editor, "dispatches 'invalid' event on editor")
     assert.equal(submitEvent, null, "does not dispatch a 'submit' event")
   })

src/trix/elements/trix_editor_element.js

@@ -228,7 +228,7 @@ class ElementInternalsDelegate {
     const { required, value } = this.element
     const valueMissing = required && !value
     const customError = !!customValidationMessage
-    const input = Object.assign(document.createElement("input"), { required })
+    const input = makeElement("input", { required })
     const validationMessage = customValidationMessage || input.validationMessage
 
     this.#internals.setValidity({ valueMissing, customError }, validationMessage)