With AndroidStudio ArcticFox the approach is as follows:
In the toolbar under the run configuration select "Edit Configurations"
Select (+) to add a new configuration and choose Gradle:
On the configuration page:
give the configuration a Name (Run all unit tests)
Choose your root project as Gradle Project
add the TaskscleanTestDebugUnitTest testDebugUnitTest (you can copy that from an existing configuration for unit tests, but make sure to remove the module prefix (e.g. :app:))
add --tests "*" as Arguments (apparently that tells AS to show the results as TestResults)
Select and run your new configuration. The Results will be shown in the run window: