selftests: pidfd: skip test if unshare fails with EPERM

Similar to how ENOSYS causes a skip if pidfd_send_signal is not present,
we can do the same for unshare if it fails with EPERM.  This way, running
the test without privileges causes four tests to skip but no early bail out.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>