Updated tests for the new behaviour of SymbolicUtils simplify function

[Bumblebee00]

Checklist

• [x ] Appropriate tests were added
• [x ] Any code changes were done in a way that does not break public API
• [ x] All documentation related to code changes were updated
• [ x] The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• [x ] Any new documentation only uses public API

Additional context

simplify from SymbolicUtils is gonna improve. Previously

julia> simplify(x^0.5)
x^0.5

But now

julia> simplify(x^0.5)
sqrt(x)

This is going to break tests in ModelingToolkit/test/changeofvariables.jl because on line 138, @test noise_eqs(new_sys)[2, 2] === value(substitute(2*α*y, backward_subs[2])) is gonna be avaluated as 2sqrt(w(t))*α === 2(w(t)^0.5)*α returning false, because new_sys comes from change_of_variables function that contains a simplify call.

I tought of several options to fix this test:

• changing line 138 to @test noise_eqs(new_sys)[2, 2] === value(simplify(substitute(2*α*y, backward_subs[2]))) (what i did in this pr)
• chagning line 138 to simplify(noise_eqs(new_sys)[2, 2] - value(substitute(2*α*y, backward_subs[2])))===0
• changing line 128 above to backward_subs = [x => exp(z), y => sqrt(w), u => exp(v)]

all of these work. I'd like some feedback :)