test: Fix UTF-8 JSON tests in Python 3

test: Fix UTF-8 JSON tests in Python 3

test_expect_equal_json uses json.tool from the system Python.  While
Python 2 wasn't picky about the encoding of stdin, Python 3 decodes
stdin strictly according to the environment.  Since we set LC_ALL=C
for the tests, Python 3's json.tool was assuming stdin would be in
ASCII and aborting when it couldn't decode the UTF-8 characters from
some of the JSON tests.  This patch sets the PYTHONIOENCODING
environment variable to utf-8 when invoking json.tool to override
Python's default encoding choice.