Skipping some tests by adding the usual jython conditional test around:

Skipping some tests by adding the usual jython conditional test around:

- the repr of unicode. Jython only add the u'' if the string contains char
  values > 255.
- A unicode arg to unicode() is perfectly valid in jython.
- A test buffer() test. No buffer() on Jython

This closes patch "[ #490920 ] Jython and test_unicode".