[User experience] add UX test plan (#212272)

## Summary

Resolves #211063

Adds a small manual test plan for the UX app.

The UX app is quite small and straightforward so the test plan is pretty
basic.

x-pack/solutions/observability/plugins/ux/dev_docs/test_plan.md

@@ -0,0 +1,22 @@
+# Manual test plan
+
+## Data generation
+
+User experience test data is available in the edge-oblt cluster. To connect to it via cross-cluster search in a development environment, you can utilize [oblt-cli](https://studious-disco-k66oojq.pages.github.io/user-guide/cluster-create-ccs/) with the edge-oblt template. You can interact with oblt-cli directly in your terminal or via the [slack integration](https://studious-disco-k66oojq.pages.github.io/user-guide/cluster-create-ccs/#slack). The slack integration is preferred for productivity.
+
+Oblt-cli will provide kibana.yml configuration that can be in `config/kibana.dev.yml`. This configuration will replace your existing `kibana.dev.yml` configuration. Be sure to uncomment the field `elasticsearch.ignoreVersionMismatch: true`, as the elasticsearch version often conflicts with the Kibana version.
+
+## Feature testing
+
+The user experience app is a single page containing a collection of dashboards for data from for the Elastic APM Real User Monitoring (RUM) JavaScript Agent.
+
+Because the whole app is a single page, it's quite straightforward to test. Simply interact with every element on the page. This includes
+
+1. Changing the time range on the time picker
+2. Testing all filter dropdowns
+3. Adding a url search to the url search bar
+4. However oven the ? icon to view the tooltips
+5. Use the breakdown feature on the page load distribution and total page view visualizations
+6. Click on a specific country in the page load by country visualization to test filtering from the visualization
+7. Click on a piece of the pie chart from the Vistor breakdowns to test filtering from the visualization
+8. Navigate through the console errors list