[llvm-ar][test] Move MRI tests from "llvm/test/Object/"

llvm/test/Object/ contains tests for the ArchiveWriter library, however
support for MRI scripts is found in llvm-ar and not the library. This
diff moves the MRI related tests and removes those that are duplicates.

Differential Revision: https://reviews.llvm.org/D68038

llvm-svn: 372973