added tests/fixture-generators/testdata-500k.py
added tests/fixture-generators/testdata-500k.py

writes 500k rows to a table, where each row represents a track in an artist's
library for a single day. the play count is stored in the cfq 'unit:scrobbles'.

zohmg/zohmg,zohmg/zohmg,zohmg/zohmg,zohmg/zohmg