Mass replacing the common code of all tests
```
echo "Fixing $1"
ed "$1" <<'EOF'
1,/Testing/d
i
from render_layer_common import *
import unittest
import os
import sys
sys.path.append(os.path.dirname(__file__))
.
w
q
EOF
```
Using line-width of 120
Some tests may break Blender, which makes the entire unittest routine to fail.
They are now I isolate the tests into individual files
Kudos to Sybren Stüvel and Sergey Sharybin for the advice.
Note: at the moment test_link.py is failing (since a41bbfb7)
