Add unit tests

Author: sanshi42

tests/samples/invalid_formula.xls

@@ -0,0 +1,14 @@
+import unittest
+
+import xlrd
+
+from .helpers import from_sample
+
+
+class TestInvalidFormula(unittest.TestCase):
+    def test_evaluate_name_formula_with_invalid_operand(self):
+        xlrd.open_workbook(from_sample('invalid_formula.xls'))
+
+
+if __name__ == '__main__':
+    unittest.main()