Testing unicode is great, but we appear to have had a couple
of cases where we were using strings that weren't valid UTF8.
Weirdly, other instances of these strings work -- I think because
they're in raw quotes (backticks) rather than strings. Anyway,
this is what SonarCloud fusses about.
(cherry picked from commit ab543adbe3)
* removes unused filesize function
* removes ioutil usage
* updates ioutil.ReadAll to io.ReadAll
* updates ioutil.TempFile to os.CreateTemp
* updates ioutil.TempDir to os.MkdirTemp
* updates ioutil.ReadAll to os.ReadAll
* update ioutil.WriteFile to os.WriteFile
* updates ioutil.Discard to io.Discard
* updates ioutil.ReadDir to os.ReadDir where applicable
* removes unused code in idk
* creates type to use for context value keys
* replaces assert.Nil with assert.NoError for error checks
