Today, mochitest, reftests and xpcshell use RemoteAutomation to start/stop GeckoView TestRunnerActivity, among other functions, whereas marionette and web-platform-tests use FennecRunner*. Both RemoteAutomation and FennecRunner are backed by ADBDevice.
Ideally, we would be maintaining only one library to manage a remote browser process. There is perhaps an opportunity to refactor RemoteAutomation to split it up into different services that all test harnesses could use.
* To be clear, FennecRunner is not actually specific to Fennec