Prepare for v1.7.0

[rkistner]

1. Fix the versions of @powersync/service-core - these will now be versioned together @powersync/service-image. That basically means that the version of @powersync/service-core would be bumped even if there are only changes in the service image.
2. Bump to v1.6.999, so that the next version will be 1.7.0, aligning with our cloud image versions.
3. Sneak in some test stability improvements and fixes to warnings.

[changeset-bot]

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Latest commit: b74269c

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