Well, you have couple of options:
- Don't test it (let the user test).
- Test it yourself.
Each has its consequences. Each requires zero to many efforts.
If you let user test it, you are risking annoying the user and eventually tarnishing your brand/trust. Even opensource project has threshold to tolerating bugs. The worst case, the user will leave the project/product for better alternative.
If you choose to test it yourself then you need to know how your app should benefit the user. You need to know what your app should do and must not do. You need to own it more than just building it.
You will need to cross the borders. Not only from developer realm to testing realm, but also into product design realm. You need to figure out not only how to implement dropdown UI, but also why it needs to be a dropdown. Why not listbox, why not textbox, what is that you want the user to accomplish.
In the alternate reality, you have a big team with specific roles. You have a team of testers. You have a product designer/manager. Finally you can focus on building things. The boundaries are clear and you are afraid to cross them.
This alternate reality, while looks perfect with its roles, is full of pitfall. The boundaries does not incentives team member to cross them which eventually lead to disconnects.
Your team delivers the product late because it was buggy and needs multiple fixes. When the product is finally out, it requires hotfixes as it doesn't actually address critical use case.
As much as we want clear separation of roles and responsibility, each roles still need to stretch a bit beyond its boundary. Each roles need to make sure it gets the right/complete input and delivers the right/complete output. It won't happen without stretching, reaching to adjacent roles.
To practice this mentality, imagine you don't have tester. Or, imagine you don't have developers. Pick any role and imagine what you would do without them. You will need to own their role in their place. You'll know how to stretch.
Have you stretch beyond your boundaries?