Move rust tests to standalone files

[mccalluc]

With tests in separate files, they code they test won't need to be rebuilt as often.

• Fix #1513
• Branched from #1526: Review that first

Draft

[mccalluc]

This is most of it:

$ egrep 'cfg.*test' -r src -A 1 | grep 'mod.rs' | grep -v cfg | grep -v 'mod test;' | wc -l
       9
$ git checkout main
Switched to branch 'main'
Your branch is up to date with 'origin/main'.
$ egrep 'cfg.*test' -r src -A 1 | grep 'mod.rs' | grep -v cfg | grep -v 'mod test;' | wc -l
      63

• I narrowed the scope to only do this refactor on mod.rs files (with the exception of error.rs which I happened to do first). If we carry it through to all files, then it's effectively saying that every file should be mod.rs in its own directory, and that might be too much of a good thing.
• Along the way, I renamed to test, although that causes some problems: there are some public modules with utilities, that are then * imported, and we would now have name conflicts.
• In some cases there are multiple [cfg(test)] blocks with different sets of extra features. For now, I've just moved one over to test.rs.
• An easy mistake to make would be to clear out the test code in file, but not move it all to the new file, but I think the test count is the same here and in main: 216.

Since this touches so many files, it's likely to pick up merge conflicts ... but conversely, it might introduce merge conflicts in other PRs which are higher priority.