Construct tests for new linear module in #303

[jamesturner246]

Tests will be required for the new linear model module, described in #385.

At a minimum, we should test:

• The module loads and runs without error
• The initialisation method is computationally correct for various inputs and coefficients
• The update method is computationally correct for various inputs and coefficients
• The new risk factor values are correctly stored in the population afterwards