Add test of interior boundary conditions

tests/meshes/square_with_embedded_line.geo

@@ -0,0 +1,44 @@
+//
+// Labels on lines are the mesh markers
+//               4
+//
+//       6-------5-------4
+//       |       |       |
+//       |       |       |
+//       |       |       |
+//     1 |       5       | 2
+//       |       |       |
+//       |       |       |
+//       |       |       |
+//       1-------2-------3
+//
+//               3
+
+
+Point(1) = {0, 0, 0, 0.1};
+Point(2) = {0.5, 0, 0, 0.1};
+Point(3) = {1, 0, 0, 0.1};
+Point(4) = {1, 1, 0, 0.1};
+Point(5) = {0.5, 1, 0, 0.1};
+Point(6) = {0, 1, 0, 0.1};
+
+Line(1) = {1, 2};
+Line(2) = {2, 3};
+Line(3) = {3, 4};
+Line(4) = {4, 5};
+Line(5) = {5, 6};
+Line(6) = {6, 1};
+
+Line Loop(1) = {1, 2, 3, 4, 5, 6};
+Plane Surface(1) = {1};
+
+Line(7) = {2, 5};
+Line{7} In Surface{1};
+
+Physical Surface(1) = {1};
+
+Physical Line(1) = {6};
+Physical Line(2) = {3};
+Physical Line(3) = {1, 2};
+Physical Line(4) = {4, 5};
+Physical Line(5) = {7};