fix: add test scripts to sample snap

[mikesposito]

Explanation

The following scripts were missing from sample-snap's package.json file:

• test:clean
• test:verbose
• test:watch

This was causing yarn test to call itself recursively, instead of running tests like yarn workspace @metamask/sample-snap test would.

References

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
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

[mikesposito]

test scripts requirements have been aligned so that snap and non-snap packages need to follow the same constraints