test: test helper functions

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What's new in this PR

A testHelpers.ts file, which contains:

• helper functions that instantiate all relevant classes with simplified values
• TSdoc for each function

Why

When writing a test for the Lifetime class, I realised that it needs all four tenure classes as well as the Property class, which contain sub-classes too. Not only was it too messy to instantiate all of that in the Lifetime.test.ts file, but I realised that we were creating many instances of these classes and their sub-classes across the test files, and thought it would be straightforward for all testing to have a set of helper functions that instantiate them with default values.

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Following dev call today, just noting that Claude suggested we go with this route because

• we are dealing with complex class instances and these functions will instantiate actual instances of our production code
• using functions means we won't accidentally stub behaviour that we want to test
• if class interfaces change, TypeScript will catch errors

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This branches from oz/resale-values which branches from oz/depreciation-update (where the test issue should be resolved).