(Again, apologies if this isn't the appropriate channel to suggest Arcanist enhancements like this.)
I'm trying to make a unit test engine and give it additional options via my .arcunit file, but there isn't currently a way to do that.
Add getEngineConfigurationOptions/setEngineConfigurationValue to ArcanistUnitTestEngine, shamelessly copied from ArcanistLinter, and call those functions from ArcanistConfigurationDrivenUnitTestEngine (you guessed it, shamelessly copied from ArcanistConfigurationDrivenLintEngine).