e2e: scope index.cy.js tree assertions to the treeitem

cy.contains("preview")/("Emails") matched the label text directly, so
wrapping the label (e.g. a truncating <div>, as in #484) made the
assertions target the wrapper instead of the role="treeitem" element and
fail on the role/aria-expanded attrs. Scope with
cy.contains('[role="treeitem"]', ...) so the checks stay on the treeitem.
Verified locally: passes both with and without the wrapper.

Author: psugihara

e2e/mailing_tests/cypress/e2e/index.cy.js

@@ -6,10 +6,12 @@ describe("index page tests", () => {
   it("should redirect index to previewFunction with tree", () => {
     cy.location("pathname").should("eq", "/previews/Welcome/preview");
 
-    cy.contains("preview")
+    // Scope to the treeitem rather than the label text so the assertions stay
+    // valid when the label is wrapped (e.g. in a truncating element).
+    cy.contains('[role="treeitem"]', "preview")
       .should("have.attr", "aria-selected", "true")
       .should("have.attr", "role", "treeitem");
-    cy.contains("Emails")
+    cy.contains('[role="treeitem"]', "Emails")
       .should("have.attr", "aria-expanded", "true")
       .should("have.attr", "aria-selected", "false")
       .should("have.attr", "role", "treeitem");