fixup! Add some tests and Travis integration

test_mazes.py

@@ -1,6 +1,7 @@
 """
 Basic tests to ensure maze files are properly formatted.
 """
+from collections import Counter
 from pathlib import Path
 import re
 
@@ -114,6 +115,15 @@ def test_boundaries(maze_file):
     assert all(h == '---' for h in north_boundary + south_boundary)
 
 
+@all_mazes
+def test_starting_cell_unique(maze_file):
+    """
+    The starting cell, if marked, must be unique.
+    """
+    rows = read_maze(maze_file)
+    assert Counter(''.join(rows))['S'] in (0, 1)
+
+
 @competition_mazes
 def test_starting_cell(maze_file):
     """