Try to make certain integration tests more reliable

Some tests are rather flaky, because visibilities may be determined to
be false by Playwright. Maybe, the workaround to first wait for these
elements helps.

test/integration.spec.js

@@ -373,8 +373,9 @@ describe("Integration tests with their own app instance each", () => {
         const DRAG_DISTANCE = 50
 
         async function displaySeparator() {
-            await clickMenuItem(toc.SHOW_FOR_ALL_DOCS_MENU_ID)
             const separatorLocator = _page.locator(mocking.elements.separator.path)
+            await clickMenuItem(toc.SHOW_FOR_ALL_DOCS_MENU_ID)
+            await separatorLocator.waitFor()
             assert(await separatorLocator.isVisible())
             return separatorLocator
         }
@@ -490,8 +491,10 @@ describe("Integration tests with their own app instance each", () => {
         const settings = require("../app/lib/settingsMain")
 
         async function opendDialog() {
+            const settingsDialogLocator = _page.locator(mocking.elements.settingsDialog.path)
             await clickMenuItem(settings.SETTINGS_MENU_ID)
-            assert(await _page.locator(mocking.elements.settingsDialog.path).isVisible())
+            await settingsDialogLocator.waitFor()
+            assert(await settingsDialogLocator.isVisible())
             assert(!(await menuItemIsEnabled(settings.SETTINGS_MENU_ID)))
         }