Update variable_utils.jl

Author: YingboMa

test/variable_utils.jl

@@ -1,8 +1,9 @@
 using ModelingToolkit, Test
+using SymbolicUtils: <ₑ
 @parameters α β δ
 expr = (((1 / β - 1) + δ) / α) ^ (1 / (α - 1))
-ref = [β, δ, α]
-sol = Num.(ModelingToolkit.get_variables(expr))
+ref = sort([β, δ, α], lt = <ₑ)
+sol = sort(Num.(ModelingToolkit.get_variables(expr)), lt = <ₑ)
 @test all(simplify, sol[i] == ref[i] for i in 1:3)
 
 @parameters γ