Fix tests for cover art vs formats, they couldn't fail.
Reduce cover art vs formats test code redundancy.
Fix test_ape failure due to incorrect APEValue type (auto)detection.

Force APEValue type on save to mutagen.apev2.BINARY, it is needed because
mutagen is trying to detect type, and set it to TEXT, and when it is tested
on load (apev2.py _load()), it doesn't detect cover tag because it isn't of
the BINARY type as expected.
Fix test_mp4 failure AssertionError: 'image/jpeg' != 'image/png'

value is already a MP4Cover, calling it again just set the format to default
JPEG, causing this issue on load.
Merge branch 'tests_formats_fix' of https://github.com/zas/picard