fix boundary_condition_slip_wall for 2D TreeMesh

[ranocha]

Closes #1190

CC @andrewwinters5000

The problem was that this is an instance where the boundary flux does not simply change its sign when changing the sign of the normal direction. Thus, we need to use the same logic based on the direction as the StructuredMesh.

[ranocha]

Locally, the test from #1190 runs fine for me. When we merge this, it will be covered by tests in #1165

[jlchan]

Thanks for the quick fix!

[codecov]

Codecov Report

Merging #1191 (8b299be) into main (2467350) will increase coverage by 19.73%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main    #1191       +/-   ##
===========================================
+ Coverage   77.05%   96.78%   +19.73%     
===========================================
  Files         310      310               
  Lines       24241    24284       +43     
===========================================
+ Hits        18677    23501     +4824     
+ Misses       5564      783     -4781     

Flag	Coverage Δ
unittests	96.78% <100.00%> (+19.73%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/equations/compressible_euler_2d.jl	99.01% <100.00%> (+12.08%)	⬆️
src/callbacks_step/amr_dg2d.jl	100.00% <0.00%> (+0.63%)	⬆️
src/solvers/dgsem_p4est/containers.jl	95.59% <0.00%> (+0.74%)	⬆️
src/equations/acoustic_perturbation_2d.jl	100.00% <0.00%> (+0.78%)	⬆️
src/equations/ideal_glm_mhd_3d.jl	100.00% <0.00%> (+0.92%)	⬆️
src/auxiliary/special_elixirs.jl	98.02% <0.00%> (+0.99%)	⬆️
...discretization/semidiscretization_euler_gravity.jl	96.45% <0.00%> (+1.02%)	⬆️
src/solvers/dgmulti/dg.jl	98.90% <0.00%> (+1.10%)	⬆️
src/callbacks_step/time_series_dg2d.jl	100.00% <0.00%> (+1.69%)	⬆️
src/solvers/dgsem_tree/containers_2d.jl	96.32% <0.00%> (+2.03%)	⬆️
... and 90 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2467350...8b299be. Read the comment docs.

[andrewwinters5000]

Thanks for fixing this! We can also keep this sign flip in mind if / when this BC for TreeMesh is implemented in 3D. Right now, the 3D version is only implemented using the normal direction, i.e., for the P4estMesh.