added test for SFX 7z archive

Hello. I need to extract SFX with it. I have made a test for it and tried to implement it.
I had added a search for a signature (naive, but maybe it will be better to implement some better algorythm) and then i had prolems.
The first one is that there is some difference between SFX and not-sfx archive which causes headers and hashsumm corruption.
The second one s that the architecture of this library is a bit strange, so it will be good to change it.
For example we can use Builder pattern : divide the process into generic stages such as finding a signature and recognizing the type using it is the one stage, reading header is another, ... etc.