Fixed test and converted to unittest format.

Checking // would call floor division but did not test that
true division had become the default with 'from __future__ import division'.

Lib/test/output/test_future

 test_future
 6
 6
-2
 SyntaxError badsyntax_future3 3
 SyntaxError badsyntax_future4 3
 SyntaxError badsyntax_future5 4