pt/test_panel_brightness: Test max_brightness as well

This makes sure we don't regress on brightness conversion

pytests/test_panel_brightness.py

 import os
 from . import boardtype
+from . import sysfs
 
 
 def test_panel_brightness():
     "Check if panel brighness range"
     path = "/sys/class/backlight/backlight-dsi"
     upper = 228 if boardtype.is_librem5() else 100
+    mb_path = os.path.join(path, 'max_brightness')
+
+    sysfs.check_content_in_glob("{}\n".format(upper), mb_path)
     f = open(os.path.join(path, 'brightness'))
     assert int(f.read()) in range(0, upper + 1)