Add comprehensive tests for chapter code snippets

Tests verify Devito code examples from:
- devito_abstractions.qmd: Grid, Function, TimeFunction, 2D diffusion
- boundary_conditions.qmd: Dirichlet BCs, subdomain method, complete solver
- advec1D_devito.qmd: Upwind, Lax-Wendroff, Lax-Friedrichs schemes

All 14 new tests pass, ensuring code snippets in the book compile
and produce correct results.

Author: ggorman