ols: add test mode support

ols allows both external and internal test patterns at ~20khz
which are helpful for unit tests and demos.

pattern=internal -> route pattern internally to all 32 pins
                      (input otherwise disabled)

pattern=external -> generates pattern on unbuffered pins 16:31
                      (which can be looped back to the buffered pins 0:15)

Signed-off-by: Matt Ranostay <redacted>