Needed tests for stdin/stdout input/output

[bskinn]

Keeping notes on needed tests for #131 in a different chain from that issue.

• Need test for data coming in over stdin, to see whether it's possible to query terminal for whether to overwrite or not.
  • If not, can just default to not-overwrite, with message instructing to use -o if overwrite is desired.
• Testing stdin with all three formats (fail on zlib)
• Testing stdout with all three formats (fail on zlib)
• Testing stdin --> stdout
• Testing presence/absence of json_data["metadata"]["url"] when using sphobjinv co json from URL/non-URL source
• Test that JSON generated w/, w/o -u loads correctly back into soi, both via disk and via stdio