[llvm-objcopy] Move elf-specific tests into subfolder

In this diff the elf-specific tests are moved into the subfolder llvm-objcopy/ELF
(the change was discussed in the comments on https://reviews.llvm.org/D54674).
A separate code reivew wasn't sent for this change 
since Phabricator is failing to create such a large diff.

Test plan: 
make check-all
make check-llvm-tools
make check-llvm-tools-llvm-objcopy

llvm-svn: 347958