shared/json: make it possible to specify source name for strings too, add tests

shared/json: make it possible to specify source name for strings too, add tests

The source would be set implicitly when parsing from a named file. But
it's useful to specify the source also for cases where we're parsing a
ready string. I noticed the lack of this API when trying to write tests,
but it seems generally useful to be specify a source name when parsing
things.