Make CI tests a bit less expensive

For example, here is a list of examples that are relatively expensive on Windows (2D)

- `examples\2d\elixir_advection_amr.jl`, 93.2s
- `examples\2d\elixir_advection_amr_nonperiodic.jl`, 47.3s
- `examples\2d\elixir_hypdiff_nonperiodic.jl`, 38.9s
- `examples\2d\elixir_euler_shockcapturing.jl`, 46.5s
- `examples\2d\elixir_euler_blast_wave_amr.jl`, 62.8s
- `examples\2d\elixir_euler_sedov_blast_wave.jl`, 61.1s
- `examples\2d\elixir_euler_positivity.jl`, 57.6s
- `examples\2d\elixir_mhd_alfven_wave.jl`, 46.6s
- `examples\2d\elixir_mhd_alfven_wave_mortar.jl`, 74.7s
- `examples\2d\elixir_mhd_orszag_tang.jl`, 45.8s
- `examples\2d\elixir_lbm_lid_driven_cavity.jl`, 40.6s
- `examples\2d\elixir_mhd_rotor.jl`, 192s
- `examples\2d\elixir_mhd_blast_wave.jl`, 158s

and Ubuntu (3D)

- `examples/3d/elixir_advection_mortar.jl`, 40.7s
- `examples/3d/elixir_hypdiff_nonperiodic.jl`, 51.9s
- `examples/3d/elixir_euler_amr.jl`, 111s
- `examples/3d/elixir_euler_shockcapturing.jl`, 67.1s
- `examples/3d/elixir_euler_sedov_blast_wave.jl`, 72.0s (although it's using only 5 time steps)
- `examples/3d/elixir_eulergravity_eoc_test.jl`, 44.2s (although it's using only 9 time steps)