python: Add python script to check hpack-test-case json files

See https://github.com/Jxck/hpack-test-case for the json file
format.

python/hpackcheck.py

+#!/usr/bin/env python
+#
+# This script reads json files given in the command-line (each file
+# must be written in the format described in
+# https://github.com/Jxck/hpack-test-case). And then it decompresses
+# the sequence of encoded header blocks (which is the value of 'wire'
+# key) and checks that decompressed header set is equal to the input
+# header set (which is the value of 'headers' key). If there is
+# mismatch, exception will be raised.
+#
+import sys, json
+from binascii import a2b_hex
+import nghttp2
+
+def testsuite(testdata):
+    if testdata['context'] == 'request':
+        side = nghttp2.HD_SIDE_REQUEST
+    else:
+        side = nghttp2.HD_SIDE_RESPONSE
+
+    inflater = nghttp2.HDInflater(side)
+
+    for casenum, item  in enumerate(testdata['cases']):
+        if 'header_table_size' in item:
+            hd_table_size = int(item['header_table_size'])
+            inflater.change_table_size(hd_table_size)
+        compressed = a2b_hex(item['wire'])
+        # sys.stderr.write('#{} WIRE:\n{}\n'.format(casenum+1, item['wire']))
+        # TODO decompressed headers are not necessarily UTF-8 strings
+        hdrs = [(k.decode('utf-8'), v.decode('utf-8')) \
+                for k, v in inflater.inflate(compressed)]
+
+        expected_hdrs = [(list(x.keys())[0],
+                          list(x.values())[0]) for x in item['headers']]
+        hdrs.sort()
+        expected_hdrs.sort()
+        if hdrs != expected_hdrs:
+            sys.stderr.write('FAIL case#{}\n'.format(casenum + 1))
+            sys.stderr.write('expected:\n')
+            for k, v in expected_hdrs:
+                sys.stderr.write('{}: {}\n'.format(k, v))
+            sys.stderr.write(', but got:\n')
+            for k, v in hdrs:
+                sys.stderr.write('{}: {}\n'.format(k, v))
+            raise Exception('test failure')
+    sys.stderr.write('PASS\n')
+
+if __name__ == '__main__':
+    for filename in sys.argv[1:]:
+        sys.stderr.write('{}\n'.format(filename))
+        with open(filename) as f:
+            input = f.read()
+        testsuite(json.loads(input))