(core) tweak duplicate document test to be more reliable

Summary:
When running as a deployment test, there are some timing issues
in the DuplicateDocument test. These tweaks appear to resolve
them.

Test Plan: checked with staging_deployment test target

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3638

test/nbrowser/DuplicateDocument.ts

@@ -25,7 +25,7 @@ describe("DuplicateDocument", function() {
     assert.equal(await driver.find('.test-modal-confirm').getAttribute('disabled'), 'true');
     // As soon as the textbox is non-empty, the Save button should become enabled.
     await nameElem.sendKeys('a');
-    assert.equal(await driver.find('.test-modal-confirm').getAttribute('disabled'), null);
+    await driver.findWait('.test-modal-confirm:not(:disabled)', 5000);
 
     // Save a copy with a proper name.
     await gu.completeCopy({destName: 'DuplicateTest1', destWorkspace: 'Test Workspace'});