MAINT: Use trading day increment instead of timedelta in test factory.

In the test factory creation of returns, the date creation was using
a timedelta of one day instead of incrementing by trading days.

Working towards changing risk module behavior which would leverage
the trading day map, but tests fail because non-trading days are
created.

Remove `factory.create_returns`, moving uses of that function to us
`factory.create_returns_from_period`, since the number of days input
for `create_returns` was more difficult to use when specifying ranges
over arbirtray dates.