Test the Python C JSON extension

The C JSON parser was added quite a while ago, but unless you
configure with --enable-shared and have the Python 2/3 development
libraries installed, and the resulting python-ovs module installed,
'make check' won't actually test it.

This patch changes Python-based tests to run from the
$builddir/python directory and makes the tests configurable to use
both JSON backends. There are some unicode failures in the C JSON
extension that I left unfixed in this patch to make it easy to
show run the new tests on broken code. The next patch in this set
works around the issue.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Lucas Alvares Gomes <lucasagomes@gmail.com>

python/ovs/json.py

@@ -21,10 +21,13 @@ import sys
 
 import six
 
+PARSER_C = 'C'
+PARSER_PY = 'PYTHON'
 try:
     import ovs._json
+    PARSER = PARSER_C
 except ImportError:
-    pass
+    PARSER = PARSER_PY
 
 __pychecker__ = 'no-stringiter'
 
@@ -91,10 +94,9 @@ class Parser(object):
     MAX_HEIGHT = 1000
 
     def __new__(cls, *args, **kwargs):
-        try:
+        if PARSER == PARSER_C:
             return ovs._json.Parser(*args, **kwargs)
-        except NameError:
-            return super(Parser, cls).__new__(cls)
+        return super(Parser, cls).__new__(cls)
 
     def __init__(self, check_trailer=False):
         self.check_trailer = check_trailer