add CantileverBeam example to testsuite

tests/examples/cantilever_beam/definition.json

+{
+  "executable": [
+    "examples",
+    "ae108-examples-CantileverBeam"
+  ],
+  "compare_stdout": "numeric",
+  "mpi_processes": [
+    1,
+    3
+  ]
+}
\ No newline at end of file

tests/examples/cantilever_beam/references/stdout.txt

+Vec Object: 1 MPI processes
+  type: seq
+0.
+0.
+0.
+0.
+0.
+0.
+0.
+-0.0682796
+0.
+0.
+0.
+-0.101859