So you are assigned to write tests which you consider tedious, but you want to work on the main code base presumably on features. Here are my thoughts:

1. Always deliver the goods: Execute your assigned work to the highest quality. To help with mindset, instead of thinking of your work in terms of "effort", think in terms of "contribution". Great tests contribute substantially to the quality of the product and prevent future regression. Find ways to deliver some bonus improvements, for example: improving coverage, improving test speed. Demonstrate that you create value in the team beyond what is assigned. Again thinking in terms of your contribution helps ie. contribute above and beyond what is asked.

2. Document the value you add and highlight it to your team and manager. Don't be passive about it. Building a reputation of quality and reliability as an engineer will help you navigate this project and beyond.

3. Be direct and clear about asking for work that you are interested in. Don't be passive. Passive is weak. Top performers are not passive, they are clear about their goals and direction and go after it directly.

Hope this helps. Good luck!