test: use new account-api/mocks module

[ccharly]

Explanation

Using the new @metamask/account-api/mocks mocks to remove duplicated test account definitions.

References

N/A

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 communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes