tests: str(datafiles) instead of a longer thing

Replace some popular copypasta.

This important-looking invocation:

os.path.join(datafiles.dirname, datafiles.basename)

is equivalent to this shorter invocation:

project = str(datafiles)

It seems like it's very popular copypasta, replace it with the shorter one thus:

# Use 'gsed' or 'sed' etc. as appropriate for your system.
git config --global alias.sub '!f() { git grep --name-only --null "$1" | gxargs --null gsed --in-place --expression "s/$1/$2/g" ; }; f'
git sub 'os.path.join(datafiles.dirname, datafiles.basename)' 'str(datafiles)'