Add test-only command to bypass build.

[scottandrewzip]

Great utility for CI. Add test-only for when you need to separate build and test steps. Our test step was taking 3 minutes after we would do a build. This was due to the shear size of our project.

[bartekpacia]

see also #1982 which where I proposed this feature

I suggest renaming from "test only" to "run"

[piotruela]

Thanks @scottandrewzip for the input.
I checked how it works and looks like the behavior of this command varies between platforms. On Android, the _execute method invokes the ./gradlew connectedAndroidTest task, which rebuilds the test by updating it. On iOS, however, the xcodebuild --test-without-building command is used, which utilizes already built instrumentation so even if we change something in the test code we won't see it.
We need to find a way to solve the disparity by making it work the same way on both platforms

[bartekpacia]

@piotruela To skip building of tests with Gradle and just run tests directly on the device, you can Orchestrator directly through adb