test: make testkit results trustworthy across WyW updates

[Anber]

• Package-specific test tasks (@linaria/atomic, react, postcss-linaria, testkit) replace the shared test definition and so were cached. A run against a new WyW build could replay an old green result. They now set cache: false, as the shared task does.
• The snapshot serializer now writes dependencies from other workspace packages (e.g. @linaria/core's dist) relative to the repository. Snapshots no longer depend on where the repository is checked out.

No snapshot changes with the current WyW.

[changeset-bot]

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Latest commit: e9113f3

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