Chart library: Improve overall test coverage

[annacmc]

Proposed changes:

• This PR adds tests (and some small additions of roles and testing IDs) to the legend and tooltip components

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Run tests for the charts package:

cd projects/js-packages/charts
pnpm test

Verify that all tests run successfully and pass.

In your Jetpack local env, navigate to the Storybook development environment:

pnpm i
cd projects/js-packages/storybook
pnpm run storybook:dev

Find JS Packages/Charts/ in the sidebar then navigate through the different charts, to check everything still works as expected.

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• 🔴 Add a "[Type]" label (Bug, Enhancement, Janitorial, Task).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

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Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Choose a review path based on your changes:
   • A. Team Review: add the "[Status] Needs Team Review" label
     • For most changes, including minor cross-team impacts.
     • Example: Updating a team-specific component or a small change to a shared library.
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     • For significant changes to core functionality.
     • Example: Major updates to a shared library or complex features.
   • C. Both: Start with Team, then request Crew
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     • Example: Refactor affecting multiple systems.
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[github-actions]

Code Coverage Summary

Coverage changed in 2 files.

File	Coverage	Δ%	Δ Uncovered
projects/js-packages/charts/src/components/legend/base-legend.tsx	10/10 (100.00%)	10.00%	-1 💚
projects/js-packages/charts/src/components/tooltip/base-tooltip.tsx	4/4 (100.00%)	50.00%	-2 💚

Full summary · JS report

[kangzj]

LGTM!