Type check test files

[colebemis]

Problem

As part of the TypeScript refactor, we're rewriting component test files in TypeScript. However, we never type check these test files during CI. So test files with type errors could still pass CI and be merged into the main branch, causing errors in consuming applications.

For example, if the type definition for the Box component is incorrect, we might not know about the error until a consuming application uses Box and encounters the error. If we type check the Box test file during CI, we'd surface the error before merging the PR.

Solution

This PR updates tsconfig.json to include test files when type checking and generating type definition files.

Impact

If any test files contain type errors, CI will fail. Since our tests (should) resemble the way components are used, we can catch type errors before they reach consumers.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/primer/primer-components/3591cf9mc
✅ Preview: https://primer-components-git-colebemis-type-check-tests.primer.now.sh

[dmarcey]

Good catch - this is a great fix to make sure that we're typing properly in @primer/components before the new versions get into consumers' hands.