document curl vs. wget

There's a comment in doc-src/install.rst explaining why the test suite uses wget instead of curl. This has come up a few times (e.g., #245) and we can't remember the reasoning for the choice.

This issue is to promote the comment to documentation. Maybe it's time for a contributor's guide? The only other developer docs we have written down that I could immediately find are doc-src/README, but those are really only sort of developer docs.