Floating-point discrepancy between Ubuntu and MacOS tests for `DGMulti` prism solver

[tristanmontoya]

In #2259, it was noted that for Wedge elements with a scalar polynomial degree (i.e. using the elixir https://github.com/trixi-framework/Trixi.jl/blob/dgmulti-mesh-io-support/examples/dgmulti_3d/elixir_advection_tensor_wedge.jl with polydeg = 3 instead of the default polydeg = (3, 4)), Ubuntu and MacOS give linf error values which differ a bit more than usually observed for double-precision arithmetic. Relaxing atol to 1e-10 gets the tests to pass, but it is still unclear what the cause of such discrepancies is; decreasing the time step helps only marginally (even with a drastically smaller CFL and/or switching to a fixed step size, the tests do not pass under the default tolerances for linf).