feat(test): harmonize AssetIndex test to use a common test specification

[paullatzelsperger]

What this PR changes/adds

Adds a common test specification for AssetIndex tests which all the implementors of AssetIndex inherit.

Why it does that

To provide a "least common denominator" for tests, i.e. a minimum subset of test scenarios for each AssetIndex.

Further notes

• the SqlAssetIndex was changed so that it now supports "upset" semantics at the store level. This was done to have consistency across all stores.
• a bug in the AssetDocument (cosmos implementation) was fixed: the createdAt field was not correctly serialised.

Linked Issue(s)

Closes #1938

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• assigned appropriate label? (exclude from changelog with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)

[paullatzelsperger]

@wolf4ood if you wanna take a look.