Mark test_svn.py as a UTF-8 source file.
Mark test_svn.py as a UTF-8 source file.

`test_svn.py` received a patch that used a Unicode character for a test,
but lacked a `coding: utf-8` at the top, breaking Python 2.7. This is a
trivial change to add the encoding information.

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools