Expand spec for scenario updater flow

[louispt1]

Bug Description

The scenario updater was not well tested in the first place, and now that we have updated to a more testable structure, we can take the opportunity to write more comprehensive tests, and assess the existing tests.

A lot of the tests that are there now were ripped from the previous ScenarioUpdater implemenation as they are still relevant for the new format. However, as a result of this issue, I noticed that several parts of the update flow and underlying models etc were lacking testing.

This issue is to write tests for those parts. From my digging, I found tests to be required for:

• Input::Cache
• Testing cache method on Input model
• Input::ScaledInputs
• ScenarioScaling.input_cache
• CalculateBalancedValues (scenario updater)
• ValidateBalance (scenario updater)
• ScenarioUpdater: test RuntimeError rescue clause
• Input::Scaler
• ShareGroups (scenario updater)
• ScenarioScaling.set_base_with

This is a long list! And there is a chance more stuff is noticed while writing these tests, or by using a coverage tool.