tests: fix false failure in cp --preserve=context test

tests: fix false failure in cp --preserve=context test

With libselinux-2.2.1-6.fc20.x86_64, kernel-3.12.6-300.fc20.x86_64
`cp --preserve=context src dst` was seen to succeed when src and
dst where on the same fixed context file system, as lsetfilecon()
returned success in this case when the context wasn't being changed.

* tests/cp/cp-a-selinux.sh: Copy from a different file system to
most likely have a different context that will test context
setting logic correctly.