I keep forgetting to make this setting when I configure a new development environment.

Visual studio Testresults setting
Visual studio Test results setting
The default setting for this is 25, meaning that visual studio will keep the 25 last test results for the specific test project. With many test-projects and especially if you work in several branches with testing the amount of storage the tests results require can be quite substantial. Especially if you run tests in win-forms applications that have a lot of graphics.