Improve curve model testing

[daniel-savu]

Is your feature request related to a problem? Please describe. Compared to the JumpModel, the loans interest rate CurveModel has very few tests.

Describe the solution you'd like Since the CurveModel is not used at all, either remove it or test it more thoroughly. https://github.com/interlay/interbtc/blob/f0a0144a0cd7e512bf9b2c55e57b12e4b3b5e9c4/crates/loans/src/rate_model.rs#L236