Allow more allocations in threaded test

[ranocha]

I just bisected this to updating Trixi.jl from v0.15.7 to v0.15.8. The explanation is that trixi-framework/Trixi.jl#2831 introduced an additional threaded loop prolong2interfaces!. Since threaded code leads to some small amount of allocations, this explains the additional allocations.

With Trixi.jl v0.15.7:

julia> u_ode = copy(sol.u[end]); du_ode = similar(u_ode); @allocations Trixi.rhs!(du_ode, u_ode, semi, 0.0)
5

julia> u_ode = copy(sol.u[end]); du_ode = similar(u_ode); @allocated Trixi.rhs!(du_ode, u_ode, semi, 0.0)
1984

With Trixi.jl v0.15.8:

julia> u_ode = copy(sol.u[end]); du_ode = similar(u_ode); @allocations Trixi.rhs!(du_ode, u_ode, semi, 0.0)
6

julia> u_ode = copy(sol.u[end]); du_ode = similar(u_ode); @allocated Trixi.rhs!(du_ode, u_ode, semi, 0.0)
2352

We clearly see that there is one additional allocation caused by one additional threaded loop.

Closes #148

[tristanmontoya]

Thanks for tracking this one down!

[ranocha]

There are even more errors than the one fixed in #153. Please fix them and proceed with merging the PRs as you see fit.

[tristanmontoya]

There are even more errors than the one fixed in #153. Please fix them and proceed with merging the PRs as you see fit.

Yes, finishing that now.