test: Use small Python script for JSON normalization instead of json.tool

json.tool does not sort or otherwise normalize the order of JSON keys
in its output, which can result in test failures on some test systems.

Instead, use a one-line Python script passed to the interpreter
directly on its command line. Use sort_keys=True for json.dump to
ensure the key order is normalized. The script works with both Python
2 and 3.

* test/test-lib.sh: Update test_expect_equal_json.