[Lens] Restore embeddable test coverage

[dej611]

Summary

Fixes #198754

Restore previous removed tests when performing the refactor.
The new tests take advantage of the new architecture to be more modular and close to the logic modules.

The data_loader tests are not just covering the re-render logic but also some expression_params logic, who in the past have proven to be the source of some bugs: specifically the tests will check that the params are correctly passed to the params logic and then stored correctly in the observable.

New mocks take advantage of the plain initializers to build some of the API, that will make it in sync with the actual implementation for future maintenance.

Checklist

Check the PR satisfies following conditions.

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)

[davismcphee]

Dismissed the Data Discovery review because it looks like there are no longer changes in our area.

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 2b4498c

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
lens	1.5MB	1.5MB	+79.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
lens	21	22	+1

Total ESLint disabled count

id	before	after	diff
lens	21	22	+1

History

• 💔 Build #264624 failed bdcda2a
• 💔 Build #263267 failed 1e518ae
• 💚 Build #262503 succeeded b805131
• 💔 Build #262491 failed 55c3a33