test.py: remove custom colors and define a color palette
test.py: remove custom colors and define a color palette

Using a standard Python module improves readability,
and allows using colors easily in other output.

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla