test-ovsdb: Workaround unicode bug in Python 2.4.x.

Run the following command on Xenserver:
PYTHONPATH=`pwd`/python/compat::`pwd`/python python ./tests/test-ovsdb.py \
parse-atoms '{"type": "string", "minLength": 2}'    \
'[""]'     '["a"]'     '["ab"]'     '["abc"]'     '["\ud834\udd1e"]'

And we get the following error:
UnicodeEncodeError: 'ascii' codec can't encode character u'\U0001d11e'
in position 23: ordinal not in range(128).

It looks like we are hitting the following bug:
http://bugs.python.org/issue2517

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-By: Ben Pfaff <blp@nicira.com>