Create mockDebug, prodDebug, prodRelease flavors

Mock build flavor

    // If you need to add more flavors, consider using flavor dimensions.
    productFlavors {
        mock {
            applicationIdSuffix = ".mock"
        }
        prod {

        }
    }

    // Remove mockRelease as it's not needed.
    android.variantFilter { variant ->
        if(variant.buildType.name.equals('release')
                && variant.getFlavors().get(0).name.equals('mock')) {
            variant.setIgnore(true);
        }
    }

Credit:
https://codelabs.developers.google.com/codelabs/android-testing