Test script changes, update contribution guide and unreleased changeset

[askoufis]

• Removing the && from the test script allows you to just run pnpm test -u instead of needing to pnpm test:rest -u
• test:rest is gone now, instead test runs all tests except sku-init, and now test:sku-init is only run on CI
• Updated the (✨new✨) contribution guide to mention the new test scripts, which I forgot to do in #962
• Changed the jest config so we use 4 workers for CI tests (ubuntu-latest GitHub agents have 4 CPUs for public repos)
• Updated the unreleased changeset from #966 to link to the sku docs about assertion removal

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: b0c8943f8494a55f97c975ac318219ef63e7a164

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This PR includes changesets to release 1 package

| Name | Type | | ---- | ----- | | sku | Minor |

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