Enhanced skip ocode tests

[gulrak]

This PR is meant to enhance the skip opcode tests, as three of them were succeeding if the skip opcode is a no-op. My changes test each skip with a skipping and a non-skipping test (originally only a non-skipping test was done in most of them) and are ordered in a way that no-op-ing through them will lead to a fail.

This should fix #28 as well.

PS: I additionally added an option to the ffmpeg call that stops an error in the log from appearing on my side, when building the tests. I hope it's universal.