Translational.Free test is underconstrained

[Keno]

This test:

ModelingToolkitStandardLibrary.jl/test/Mechanical/translational.jl

Line 32 in 0f0fae1

@test sol[s.free.f][end] ≈ 100 * 10

passes only by accident, because MTK happened to force acc.f to 0 (but it could have just as easily chosen free.f - SciML/ModelingToolkit.jl#2328 will give a warning in this case). The test is likely incorrect and should be fixed to do what is intended. It's also possible that free.f should be an observable rather than a variable.

[Keno]

cc @bradcarman