jpegxl.py: Minor cleanup. (OSGeo#11854)

- Remove extra empty lines
- Rename `outfilename` and `tmpfilename` -> `filename`
- Rename `/vsimem/out.jxl` to be specific to each test function
- Use `with open()` to load data with `.read()`
- Fix spelling: `withing` -> `within`
- Add a comment explaining the double run in `test_jpegxl_rasterio()`
- Uses tmp_vsimem

Fixes OSGeo#11833

Co-authored-by: Even Rouault <even.rouault@spatialys.com>