Fix off-by-one error in pixel color checks for rotate and flip image tests.
Change to using PNG with single pixel to ensure that errors are caught in the future, rather than lost in JPEG noise.
Props Fuegas, mikeschroder.
Fixes#46073.
git-svn-id: https://develop.svn.wordpress.org/trunk@45067 602fd350-edb4-49c9-b593-d223f7449a82
Mike Schroder