Add export tests for profile-sync-controller

[mcmire]

Explanation

I found it difficult to review changes to @metamask/profile-sync-controller as it was not completely clear to me what this package exported.

For other packages we have begun using unit tests for index.ts files to make exports more visible. This commit adds such tests.

References

(None)

Changelog

(N/A)

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes