1284960 - Infer emulator options for ./mach marionette-test and verify that conditions are met

Status: NEW

(Testing :: Marionette Client and Harness, defect, P3)

Description

[Maja Frydrychowicz :impossibus (was :maja_zf) (needinfo me)]

As a result of Bug 787203, you can run Marionette tests on Fennec in an emulator like this: 

>./mach marionette-test --emulator --app='fennec' testing/marionette/harness/marionette/tests/unit/test_click.py --avd-home /Users/mozilla/.mozbuild/android-device/avd --emulator-binary /Users/mozilla/.mozbuild/android-sdk-macosx/tools/emulator

The prerequisites in this case are:
1. an AVD is available (./mach android-emulator takes care of that)
2. Fennec is installed
3. We know where to find the emulator binary and the avd.

When in an Android build context, ./mach marionette-test should do something about the AVD and the Fennec installation (inform the user, offer to run those steps) and infer which emulator binary and AVD to use.

Other mach commands use mozrunner.devices.android_device to achieve this.