jpegxl.py: Minor cleanup.

- 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()`

Fixes #11833