[XCOFF][llvm-readobj] Move XCOFF test to XCOFF directory

Summary:
COFF and XCOFF in llvm are very different and serves different platform.
Since we have different Dumper.cpp file in llvm-readobj's
implementation, we should have separate testing directory for them too.

Reviewed By: jhenderson, DiggerLin

Differential Revision: https://reviews.llvm.org/D85675