Re: Reduce file usage in tests

[Chris Warburton]

Not quite implemented as described, but good enough.

The files used by tests have been consolidated down to just 16, and
these are now the only ones we provide in the BENCHMARKS_FALLBACK env
var when we're testing.

Since we're caching more thoroughly now, and tests are (for the most
part) using those caches, this is less of an issue.

Most tests are now either:

 - Using pure values
 - Using a few files (e.g. 2 or 3), which is fast enough
 - Using "all" 16 files, via caches