`convergence_test` does not work for elixirs using `TrixiExtension`

A convergence test like
```julia
convergence_test("examples/structured_2d_dgsem/elixir_advection_rotated.jl", 3)
```
fails with
```
ERROR: MethodError: no method matching (::Main.TrixiExtensionAdvectionRotated.InitialConditionConvergenceTestRotated)(::SVector{2, Float64}, ::Float64, ::LinearScalarAdvectionEquation2D{Float64})
The applicable method may be too new: running in world age 30027, while current world is 30035.
```

Start by running convergence_test("examples/structured_2d_dgsem/elixir_advection_rotated.jl", 3) and inspect how TrixiExtension elixirs are loaded. Trace the InitialConditionConvergenceTestRotated method involved in the reported world-age error. Done means the convergence test completes for this example without the MethodError.

Written by the indexing model from the issue text.