A hint to your code example (may be interesting for others…)
You can omit the keyword set_reference_folder by specifying the folder already during import:
Library ImageHorizonLibrary reference_folder=${CURDIR}${/}images
CURDIR is a robot internal variable that points to the current directory where the robot file is located. (I would prefer to have the images as well be within the robot folder under version control)
The slash in brackets - yes I know, looks stupid 
But this makes your test OS independent.
Apart from that: well done; Starting and closing the application under test is best done in the setup/teardown phase of the suite or test.