Welcome interstitial not displayed (#190045)

## Summary

Closes https://github.com/elastic/kibana/issues/186168

The tests fails because for some reason the welcome interstitial
animation doesn't complete before the timeout set for the target element
to be visible, it's interesting that despite waiting for the set
duration of the animation for the property, with a timeout that
quadruples the animation duration this tests fails still.

It's worth pointing out that in all referenced instances of failure in
the issue, on the second attempt the test actually passes, hence this PR
only introduces a retry to the assertion that the target element is
displayed. Furthermore this PR was ran through the flaky test runner
[see
here](https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/6699)
with success.

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### Risk Matrix

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and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes |

|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces&mdash;unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes&mdash;Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
add plenty of unit tests around this logic and document manual testing
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| Code should gracefully handle cases when feature X or plugin Y are
disabled. | Medium | High | Unit tests will verify that any feature flag
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Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

test/functional/apps/home/_welcome.ts

@@ -15,8 +15,7 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
   const PageObjects = getPageObjects(['common', 'home']);
   const deployment = getService('deployment');
 
-  // FLAKY: https://github.com/elastic/kibana/issues/186168
-  describe.skip('Welcome interstitial', () => {
+  describe('Welcome interstitial', () => {
     beforeEach(async () => {
       // Need to navigate to page first to clear storage before test can be run
       await PageObjects.common.navigateToUrl('home', undefined);

test/functional/page_objects/home_page.ts

@@ -57,9 +57,14 @@ export class HomePageObject extends FtrService {
   }
 
   async isWelcomeInterstitialDisplayed() {
-    // give the interstitial enough time to fade in
-    await new Promise((resolve) => setTimeout(resolve, 500));
-    return await this.testSubjects.isDisplayed('homeWelcomeInterstitial', 2000);
+    // This element inherits style defined {@link https://github.com/elastic/kibana/blob/v8.14.3/src/core/public/styles/core_app/_mixins.scss#L72|here}
+    // with an animation duration set to $euiAnimSpeedExtraSlow {@see https://eui.elastic.co/#/theming/more-tokens#animation},
+    // hence we setup a delay so the interstitial has enough time to fade in
+    const animSpeedExtraSlow = 500;
+    await new Promise((resolve) => setTimeout(resolve, animSpeedExtraSlow));
+    return this.retry.try(async () => {
+      return await this.testSubjects.isDisplayed('homeWelcomeInterstitial', animSpeedExtraSlow * 4);
+    });
   }
 
   async isGuidedOnboardingLandingDisplayed() {