Many tests without actual assertions

[victorb]

A lot of tests are currently not actually doing any valuable assertions on the responses from calls. Rather, it just makes sure there is no errors and that keys exists.

Example: https://github.com/ipfs/interface-ipfs-core/blob/9d91267ab7ab61e41c84eaa81c647cd630d83797/js/src/stats.js#L39-L59

In this sample, res could have all those keys set to null and the test would pass. We should at least assert the type of the value, but best would be the exact value if we can. If not exact, we should be able to assert the format instead.