[fetch] URL validation with { devel: filename }

[gadicc]

Following on from #27 (comment), if we use { devel: true }, we get a filename that's unique to everyone URL. But if we specify { devel: filename }, we can override that with an easy to understand name. The downside is that it's no longer unique per URL, which means that, we could change URL options (e.g. default options) and we'd still test against the original query.

• Validate that URL is the same before turning cache entry, warn or throw otherwise
• Fix tests that purposefully use result from another module