Replace equality comparisons with explicit assertions

Context

Many of the tests in MainVerticleTests use a comparison between an expected holdings object and an interpreted response from the API.

This makes it hard to understand what the expectations.

It also seems to cause intermittent build issues.

In that case, that build failure also caused an outage to our hosted reference environments, raising the priority of this work.

Scope

• Replace each comparison assertion in those tests with explicit assertions

Note that the API returns a successful empty response when a failure occurs. This is an intentional design decision.

It would be good to include comments (or similar) in these cases, to make that unexpected behaviour obvious to future readers.