Are you on a Unix platform? How about, first:

> perl -ne '
BEGIN { $i = 0 };
next if /^\s*$/;
s/^/mv /;
s/$/" testfile" . ++$i/e;
print' filenames.txt > renames.sh
[download]

Then look through renames.sh and make sure you're happy with it. Then on the command line:

> sh renames.sh
[download]