(core) Add missing .isPresent() to isOnTestLoginPage

Summary:
A staging test was failing because the util was throwing instead
of returning true or false.

Test Plan: N/A (fixing test)

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3301

test/nbrowser/homeUtil.ts

@@ -271,7 +271,7 @@ export class HomeUtil {
    * Returns whether we are currently on the test login page.
    */
   public async isOnTestLoginPage() {
-    return await this.driver.findContent('h1', 'A Very Credulous Login Page');
+    return this.driver.findContent('h1', 'A Very Credulous Login Page').isPresent();
   }
 
   /**