Add testing folders

[Sternbach-Software]

At least having commonTest or mobile test would be nice, and if users want to add more, they can copy the code. Alternatively, it can be an option to add in the wizard.

[terrakok]

Any JetBrains IDE suggests to make a test folder by CMD+N for any source set. I don't see a profit to have empty dirs here

[Sternbach-Software]

@terrakok For some reason, the "OK" button in that dialog never works for projects created from the wizard. They don't have to be null - you can include proof-of-concept tests that utilize platform-specific features (e.g. Android JUnit instrumentation, etc.)

[terrakok]

Try to make a test directly in the project view.