Attempt to fix flaky sidenav test (elastic#212213)

Fixes elastic#195153

## Summary

Waiting for the page to be loaded and the side nav item to exist before
clicking.

test/functional/page_objects/solution_navigation.ts

@@ -115,8 +115,10 @@ export function SolutionNavigationProvider(ctx: Pick<FtrProviderContext, 'getSer
       async clickLink(by: { deepLinkId: AppDeepLinkId } | { navId: string } | { text: string }) {
         await this.expectLinkExists(by);
         if ('deepLinkId' in by) {
+          await testSubjects.existOrFail(`~nav-item-deepLinkId-${by.deepLinkId}`);
           await testSubjects.click(`~nav-item-deepLinkId-${by.deepLinkId}`);
         } else if ('navId' in by) {
+          await testSubjects.existOrFail(`~nav-item-id-${by.navId}`);
           await testSubjects.click(`~nav-item-id-${by.navId}`);
         } else {
           await retry.try(async () => {

x-pack/test/observability_functional/apps/observability/sidenav/sidenav.ts

@@ -9,7 +9,11 @@ import expect from '@kbn/expect';
 import { FtrProviderContext } from '../../../ftr_provider_context';
 
 export default function ({ getPageObjects, getService }: FtrProviderContext) {
-  const { common, solutionNavigation } = getPageObjects(['common', 'solutionNavigation']);
+  const { common, solutionNavigation, header } = getPageObjects([
+    'common',
+    'solutionNavigation',
+    'header',
+  ]);
   const spaces = getService('spaces');
   const browser = getService('browser');
 
@@ -24,6 +28,7 @@ export default function ({ getPageObjects, getService }: FtrProviderContext) {
       // Create a space with the observability solution and navigate to its home page
       ({ cleanUp, space: spaceCreated } = await spaces.create({ solution: 'oblt' }));
       await browser.navigateTo(spaces.getRootUrl(spaceCreated.id));
+      await header.waitUntilLoadingHasFinished();
     });
 
     after(async () => {