Add gamma test options

While creating a frontend one needs to test the different types of parameters. One important parameter type that is lacking in the test backend is the gamma-table type (array of int)

This patch adds four gamma tables. Three 8-bit gamma tables, one for each color and one 12-bit gamma table for common brightness value.

These tables do not effect the returned data of the test backend. They are only there for testing reading and writing of the array values.