simpletest could use generalized screen size

[FoamyGuy]

Right now the simpletest creates a Bitmap with a fixed size 320, 240.

Adafruit_CircuitPython_ProgressBar/examples/progressbar_simpletest.py

Line 11 in 48bea36

color_bitmap = displayio.Bitmap(320, 240, 1)

Instead it could base the size on board.DISPLAY.width and board.DISPLAY.height and create the size of the progress bar dynamically as well based on some percentage of the screen width.

That way this example will run on devices with any screen size and not take up the memory of 320, 240 Bitmap if it is not needed for that device.