x86UNIX: Fix PlatformFileIOTests.FileWriteRead unit test

* The value being returned is a bool, so checking if it's equal to
  zero isn't correct.
x86UNIX: Fix PlatformStringTests.dStrcatlTest unit test

* The pointer to dst gets moved along to find the size of dst, but is
  moved along too far by one. We move it back to the right spot.
x86UNIX: Fix PlatformStringTests.dStrcmp16Test unit test

* Implement the UTF16 dStrcmp function for x86UNIX that was previously
  unimplemented.