[test] Replace symlinks in legacy_cubes by real copies This solves a "python setup.py build" issue on my machine. Copying these files should not be a big deal because they have been dropped in the "default" branch and we would not touch them in 3.26 branch.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)