A few Matroska testcases and an issue

Hi,
Finally I found time to "play" with CPPUNIT and wrote a few testcases. This push request contains them.

But, I have found a regression - currently none of the tests succeed due to the change of the "toUInt64BE" methods. They do not allow anymore to create a 64 bits number from a byte vector, that is only 4 bytes or less in size.
Can anybody suggest a method to avoid this issue?