tests: skip tests upon failure to set SELinux context

tests: skip tests upon failure to set SELinux context

On some setups the root:object_r:tmp_t context is invalid.
This does indicate a limitation in the test framework,
but for now we'll relax this to skipping the tests.
The tests still run on a Fedora 25 system for example.

* tests/cp/cp-a-selinux.sh: Upon chcon error, skip rather than ERROR.
* tests/install/install-Z-selinux.sh: Likewise.
* tests/misc/chcon.sh: Likewise.
* tests/misc/runcon-no-reorder.sh: Likewise.
* tests/misc/selinux.sh: Likewise.
* tests/mkdir/restorecon.sh: Likewise.