The core issue was the usage of `reference_file_path.file_name()`, which
provided a non-existent path if the file to be updated was in a
subdirectory.
Instead we have to provide the whole path after 'tests/ui/' as the
'filename'. This part of the path is called `test_name` in the code now.
Adding of new reference files wasn't handled correctly. It was trying to
read a file that didn't exist yet.
Instead of unwrapping, we now treat a missing reference file as empty
(`Vec::new`). This makes the following conditional work. We then also
have to re-read the reference file after it was being copied. This
second read is technically the same as in the old shell script, but
wasn't really obvious. The shell script did a `-s` test which reads the
file.
