integration: some fixes for seek tests

- Use g_list_remove_link so that ordering of seeks is not mandatory
- use g_slice allocator for SeekInfo structs
- Fix leak in freeing seek list
- Check for NULL seeks at end of test, otherwise fail and free failed seeks
tests: Remove prints

And use GST_DEBUG and friends instead
tests: integration: Remove the effect in test_seeking

It complexifies the test but this is not what we actually want in
that test
tests: integration: Add audio/video only seeking tests
tests: integration: Refactor and makew easier to add encoding profiles
tests: integration: Be more precise with namings

Fully define formats in the namings

+ Add an mp3 + h264 in mov test
tests: integration: Give the user more details about failure when checking transcoded file
tests: intergration: Add some more encoding profiles
tests: integration: Give some more information to user on errors
integration: display name of the source of error messages.