It's no doubt that Android Studio gets better with every version. Who doesn't love Instant Run? There is one thing I am sad to see change without mention. Test Artifacts are merged together by default now.
If you aren't familiar with testing or test artifacts in Android, Google themselves have created a really nice starting point answering most of your questions. Also I love to note that I am a huge fan of the Arrange-Act-Assert pattern and TDD. I would definitely check them out if you are new to testing or programming in general.
I usually run my tests at the command line via the same build task that will be run on the server. This validate that what I am working on will in fact pass on the server. However, if I want to debug through my tests in IDE I usually like to isolate which test artifact I am running. The reason being is that tests run on the regular JVM are usually faster and it means I don’t have to have a device connected to run them.
So if you are like me chances are that you would like to not have your tests ran together out of the box. In order to get back the old Android Studio behavior navigate to the Experimental tab in Preferences.
Preferences>Build, Execution, Deployment>Build Tools>Gradle>Experimental
And simply toggle off the
Enable all tests… box and apply. Voila! You now can switch between your Unit Tests and Android Instrumentation Tests.