tests-pure: fixing test-parseindex2

This is a followup to D7603 (49fa0b31ee1d) which broke
the tests for pure Python implementation. There are two
divergences between pure and C implementations:

- the pure implementation would accept only -1 as slice end,
  whereas C accepts both -1 and len(index)
- in pure Python, `headrevs` is provided by revlog.py, not
  by the index.

Differential Revision: https://phab.mercurial-scm.org/D7707

tests/test-parseindex2.py

@@ -267,11 +267,13 b' class parseindex2tests(unittest.TestCase'
         appendrev(6)
         self.assertEqual(len(index), 7)
 
-        del index[1:7]
+        del index[1:-1]
 
         # assertions that failed before correction
         self.assertEqual(len(index), 1)  # was 4
-        self.assertEqual(index.headrevs(), [0])  # gave ValueError
+        headrevs = getattr(index, 'headrevs', None)
+        if headrevs is not None:  # not implemented in pure
+            self.assertEqual(index.headrevs(), [0])  # gave ValueError
 
 
 if __name__ == '__main__':