Fix tests and remove broken gzip trailer validation

Hi, thanks for making Zlib portable. 
However, there were a couple of bugs in the port, that this pull request fixes:

* The MSTest based tests did not pass, mostly for file path issues 
* The port of ParallelDeflateOutput stream was broken, it didn't implement Close/Dispose correctly (doing new Dispose/new Close is almost always a bad idea as it breaks the LSP)
* I took the liberty to move the tests over to NUnit, this allows devs working in Xamarin Studio / Mono to run them as well
* A long standing issue with a broken Gzip trailer validation. Details and links about this can be found in the commit message