test: improve test coverage for DatasetIndexBuilder.

[jplaisted]

The other index builders need similar improvements, but after #1937 some coverage is better than no coverage.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[jywadhwani]

You may also want to add a test when snapshot contains only InstitutionalMemory aspect?

[jplaisted]

You may also want to add a test when snapshot contains only InstitutionalMemory aspect?

I thought about this, and the answer is no. Really, we should have some kind of test like that. That all aspects are intentionally handled / not handled. But adding a new aspect means that test will become out of date. So we'd really need some way to automatically generate that test or something, and it would fail as soon as you add a new aspect, requiring you to mark the new aspect as expecting to be handled or not.

Without that automation, I'm not interested in a test like that, no, because it quickly becomes stale and provides no benefit.

We can think about adding that automation later.