Specify `sbf.quiet` option within all tests where necessary rather than setting it for the full script

replace
```r
withr::local_options(sbf.quiet = TRUE)
```
on line 1 of `test-save-load.R` with
```r
withr::with_options({
  ...
})
```
inside each relevant test. For more context see [this comment](https://github.com/poissonconsulting/subfoldr2/pull/174#discussion_r3611890021).