Renamed old vaadin-testbench to -core and added new vaadin-testbench #13759

The old vaadin-testbench module is now called vaadin-testbench-core (also the directory) and
a new module called vaadin-testbench was created. The new module contains no code, but only
lists two dependencies: vaadin-testbench-core and vaadin-testbench-api

Change-Id: Ifdcf761a73e6304a27a580e98d0d7179289c0a6b