Missing unit and type stability tests

[MarcoArtiano]

After #114, PR #117 covers most of the unit and type stability tests. However, some of them are still missing. I’ll list them here

• - Type and unit tests for CovariantLinearAdvectionEquation2D
• - Type and unit tests for CovariantShallowWaterEquation2D
• - Type and unit tests for SplitCovariantShallowWaterEquation2D
• - Type tests for CompressibleMoistEulerEquations2D
• - Unit tests for CompressibleEulerEnergyEquationsWithGravity2D
• - Unit tests for CompressibleEulerEnergyEquationsWithGravity3D