add surface SMILES test strings to unit tests

Author: sevyharris

test/rmgpy/molecule/moleculeTest.py

@@ -1632,6 +1632,15 @@ def test_smiles(self):
             "CCCC",
             "O=C=O",
             "[C]#N",
+            "[X]",
+            "[X]C=C[X]",
+            "O[X]",
+            "CO[X]",
+            "[XH]",
+            "C=C[X]",
+            "CO.[X]",
+            "C#[X]",
+            "CCC(C)[X]"
         ]
         for s in test_strings:
             molecule = Molecule(smiles=s)