test: add new test case for diffusion module

Test case defines a 2D grid with closed boundaries and 1 constant cell
in the middle.

Every other cell is set to 0.

After each iteration the sum of all cells must be greater in comparison
to the previous state of field.

test/testDiffusion.cpp

@@ -1,7 +1,7 @@
 #include <bits/stdint-uintn.h>
-#include <grid/BTCSBoundaryCondition.hpp>
 #include <diffusion/BTCSDiffusion.hpp>
 #include <doctest/doctest.h>
+#include <grid/BTCSBoundaryCondition.hpp>
 #include <vector>
 
 using namespace Diffusion;
@@ -123,3 +123,43 @@ TEST_CASE(
     }
   }
 }
+
+TEST_CASE("2D closed boundaries, 1 constant cell in the middle") {
+  std::vector<double> field(N * M, 0);
+  double val = 1e-2;
+
+  BTCSDiffusion diffu = setupDiffu(N, M);
+  BTCSBoundaryCondition bc(N, M);
+
+  field[MID] = val;
+  bc(BC_INNER, MID) = {BC_TYPE_CONSTANT, val};
+
+  uint32_t max_iterations = 100;
+
+  double sum = val;
+
+  for (int t = 0; t < max_iterations; t++) {
+    diffu.simulate(field.data(), alpha.data(), bc);
+
+    CHECK_EQ(field[MID], val);
+
+    double new_sum = .0;
+
+    for (int i = 0; i < M; i++) {
+      // iterate through columns
+      for (int j = 0; j < N; j++) {
+        new_sum += field[i * N + j];
+      }
+    }
+
+    if (sum > new_sum) {
+      CAPTURE(sum);
+      CAPTURE(new_sum);
+      FAIL("Sum of concentrations is equal or lower than to previous iteration "
+           "after iteration ",
+           t + 1);
+    }
+
+    sum = new_sum;
+  }
+}