goss serve cache should separate results from output #612

Describe the feature:
Implementation detail change.

Sliced work from #609 (comment) discussion.

goss serve has a cache. Currently, this caches rendered output.

#609 introduces the possibility of different rendered output for the same validate-run.

It's important that the rendered outputs all tie back to the same instance of a validate-run.

Describe the solution you'd like

Cache the validate-run results only, and don't cache the output rendering

• ... because the hypothesis is that the output rendering is trivially fast

Describe alternatives you've considered

1. It would be more time-efficient (but cost memory) to cache in two stages

   1. the result of a validate run
   2. the rendered output

   ... but

   • it would introduce complexity to the cache
     • need to expire the output cache entries that are tied to a results-cache entry, when that expiry occurs
     • ideally want configurability options like max-cache-size
   • we don't (currently) have a convenient way to repeatably profile this to see whether
     • this would in fact be a net-positive in speed, compared to a regression
     • the extra memory is worthwhile
     • the speed is worth the memory + complexity