Add tests for non-square cells

[lrntct]

• Added regression test for solve_h: it sets known qe/qs values on a 5x5 stencil and checks the center-cell depth update matches ((qw - qe) / dx) + ((qn - qs) / dy). It also verifies the result would differ if dx and dy were mistakenly swapped.
• Added a small helper that builds a 5x5 simulation with arbitrary dx/dy. The helper keeps the initial water volume constant across runs, so the comparisons isolate cell-shape effects.
• Added test_rectangular_cells_preserve_axis_symmetry_and_bias_early_spreading. Checks top == bottom and left == right. Checks spreading is stronger along the axis with the smaller cell size.
• Added test_swapping_dx_and_dy_transposes_the_early_time_solution. Compares (dx, dy) = (20, 10) against (10, 20) and asserts the early-time water-depth field is transposed when the cell sizes are swapped.
• Resolves Add tests for non-square cells #122