refactor: reduce dependencies modelTests

[edkerk]

Description of the issue:

Currently, code/modelTests/ still has some COBRA Toolbox dependencies (1, 2, possibly more), that are not necessary given the aim of #301.

Expected feature/value/output:

Use RAVEN functions instead.

I hereby confirm that I have:

• Tested my code with all requirements for running the model
• Done this analysis in the main branch of the repository
• Checked that a similar issue does not exist already
• If needed, asked first in the Gitter chat room about the issue

[edkerk]

Is fixed as part of #305