test examples: support loading files

- add symlink (thank you @tamiko) to support loading input files
- add step-5

tests/examples/README.md

@@ -22,4 +22,12 @@ For development of new examples, we suggest first creating an empty
 
 To modify the source code, go into ``build/tests/examples/source/``
 and edit the .cc file there. Afterwards, you can update the .diff
-files by running the ``update_diffs`` target in that folder.
+files by running the ``update_diffs`` target in the folder
+``build/tests/examples``, i.e., by running ``make update_diffs``.
+
+To support loading files from the ./examples/step-XY/ directory, we
+automatically create a symlink such that
+```
+    SOURCE_DIR "/step-5/"
+```
+compiles to an absolute path pointing at ``./examples/step-5/``.

tests/examples/step-5.diff

@@ -0,0 +1,17 @@
+40a41
+> #include "../example_test.h"
+268,271c269,273
+<   solver.solve(system_matrix, solution, system_rhs, preconditioner);
+< 
+<   std::cout << "   " << solver_control.last_step()
+<             << " CG iterations needed to obtain convergence." << std::endl;
+---
+>   check_solver_within_range(std::cout,
+>                             solver.solve(system_matrix, solution, system_rhs, preconditioner),
+>                             solver_control.last_step(),
+>                             10,
+>                             200);
+327c329
+<   std::ifstream input_file("circle-grid.inp");
+---
+>   std::ifstream input_file(SOURCE_DIR "/step-5/circle-grid.inp");

tests/examples/step-5.output

@@ -0,0 +1,30 @@
+Cycle 0:
+   Number of active cells: 20
+   Total number of cells: 20
+   Number of degrees of freedom: 25
+Solver stopped within 10 - 200 iterations
+Cycle 1:
+   Number of active cells: 80
+   Total number of cells: 100
+   Number of degrees of freedom: 89
+Solver stopped within 10 - 200 iterations
+Cycle 2:
+   Number of active cells: 320
+   Total number of cells: 420
+   Number of degrees of freedom: 337
+Solver stopped within 10 - 200 iterations
+Cycle 3:
+   Number of active cells: 1280
+   Total number of cells: 1700
+   Number of degrees of freedom: 1313
+Solver stopped within 10 - 200 iterations
+Cycle 4:
+   Number of active cells: 5120
+   Total number of cells: 6820
+   Number of degrees of freedom: 5185
+Solver stopped within 10 - 200 iterations
+Cycle 5:
+   Number of active cells: 20480
+   Total number of cells: 27300
+   Number of degrees of freedom: 20609
+Solver stopped within 10 - 200 iterations