feature_tester_without_ui.py

import os
import sys
from vanilla.dialogs import getFile

lib_path = os.path.expanduser("~/Library/Application Support/RoboFont/plugins/FeaturePreview.roboFontExt/lib")
if not lib_path in sys.path: sys.path.append(lib_path)

from featurePreview import FeatureTester

font_file = getFile(messageText="Select font to test features", fileTypes=["ufo"])

try:
    font = OpenFont(font_file[0], showUI=False)
    FeatureTester(font=font)
except: pass