Investigate whether `test_fromfile_prefix_chars` is needed (currently marked as `xfail`)

[mo-gill]

During the update to python 3.12 , this test was marked as xfail as it was failing after the update.

This issue is to check whether the test is needed at all, as it is related to older Alter code.

https://github.com/MetOffice/CDDS/blob/main/cdds/cdds/tests/test_prepare/test_command_line.py#L577