As a good practice we always branch out our feature branch from the main branch (E.g. master) and when we are done with the development, we send a pull request to merge changes back into the main branch.
Due to the fact that main branch always evolving and going ahead with changes from other branches, when it comes to merging back a feature branch to the main branch, changes could break the code. While we expect team members to update their feature branches with latest changes from the main branch before sending a pull request to reduce the risk of broken code, this is not working well at all the time.
This is why introducing continues testing into the pull request process removes the risk of broken code. This simply can be implemented in VSTS by adding a Build Validation as part of Branch Policies.
Navigate to list of branches, find your main branch and then select “Branch Policies”.
On the policies screen, click on Build Validations and add your build pipeline to the branch policies. Continuous test can be one of the steps in your build pipeline. You can make this policy Required or Optional.
You can also enable Test Impact Analysis (TIA) to reduce the testing and respectively building time.
Arad Haghi Personal Blog 