Separate n_* tests that are about multiple texts in one file

[nicowilliams]

Tests like parsers/test_ccan_json/json/_test/nst_files/n_31.json (of which there are many) fail with jq only because jq permits multiple JSON texts in one input stream.

I'm not saying this is permissible JSON, just that it's an entire category of tests that jq will always fail (by design), so it would be best to separate them, kinda like i_* tests.

[nicowilliams]

E.g., test_parsing/n_structure_double_array.json ([][]), and test_parsing/n_structure_object_with_trailing_garbage.json ({"a": true} "x"). Also, parsers/test_ccan_json/json/_test/nst_files/n_22.json ("3" "4").