Write test for `ellipse()`

- Suppress output of the bitmap to the screen
- Check if the bitmap is equal to the expected bitmap (i.e. a bitmap of a particular ellipse)

Locate the ellipse() entry point and inspect how it currently outputs the bitmap. Suppress that screen output in the test, compare the resulting bitmap with an expected bitmap containing the target ellipse, and consider the work done when that comparison passes.

Written by the indexing model from the issue text.