Fix modules_are_self_consistent test

modules/units/stoney.nbt

@@ -1,5 +1,7 @@
 # See https://en.wikipedia.org/wiki/Stoney_units
 
+use physics::constants
+
 unit l_s: Length = sqrt(gravitational_constant × electron_charge^2 / 4 π ε0 speed_of_light^4)
 
 unit m_s: Mass = sqrt(electron_charge^2 / 4 π ε0 gravitational_constant)