test_parsing: Exercise an octal escape

[smcv]

In standardized JSON, this is an error. In JavaScript, it's a deprecated alternate spelling for "\u000a" (012 octal = 0x000a hex), and some parsers like the one in json-glib (when not in strict mode) accept this as an extension beyond standard JSON.

Resolves: https://github.com/nst/JSONTestSuite/issues/136