Test for alpha bit detection in VP8L chunks

[piponazo]

In #559 @skal65535 contributed a patch to fix the alpha bit detection in VP8L chunks. He provided a reproduction case that we should add in the python test suite.

[clanmills]

Wow. This dropped "below the radar". Let's see if we can get this fixed in v1.00.

[norbertwg]

I wanted to take care about this issue - but have some difficulties.
The original issue was, that a file did not open in Gimp. skal65535 found as root cause that the alpha bit was not set correct. Did I get this right?
My question now is, how this could be included in the test suite. Based on the original issue, it should be tested, if the file modified with the command skal65535 documented, can be opened by Gimp. But I do not think that this is possible - or at least have doubts if the effort to test with Gimp on different platforms is worth it.
Or is the idea to have just a test, that the flag is set correct?