Our test cases spew a lot of files and directories around $TMPDIR. Make more
thorough use of the testing package's TempDir methods to ensure these are
cleaned up.
In a few cases, this required plumbing test contexts through existing helper
code. In a couple places an explicit path was required, to work around cases
where we do global setup during a TestMain function. Those cases probably
deserve more thorough cleansing (preferably with fire), but for now I have just
worked around it to keep focused on the cleanup.
Closes#4603
Commands used (VIM):
```
:args `rg -l errors.Wrap`
:argdo normal @q | update
```
where q is a macros rewriting the `errors.Wrap` to `fmt.Errorf`.
