TST: add assertion that we will actually read a file

Author: cpcloud

pandas/io/tests/test_html.py

@@ -622,6 +622,7 @@ def read_string(self, f, encoding):
             return self.read_html(fobj.read(), encoding=encoding, index_col=0)
 
     def test_encode(self):
+        assert self.files, 'no files read from the data folder'
         for f in self.files:
             _, encoding = _lang_enc(f)
             from_string = self.read_string(f, encoding).pop()