To run all instrumentation tests, use the following command line:
$ ./gradlew connectedAndroidTest
Its precondition is to have a device connected or emulator running. If you have more than one device or emulator present, all of them will run tests.
To run the instrumentation test for the build variant, use the following command line:
$ ./gradlew connectedCompleteDebugAndroidTest
This will trigger all instrumentation tests for the Connected flavor with the Debug build type.