tests: test for setns() EINVAL regression

Verify that setns() reports EINVAL when an fd is passed that refers to an
open file but the file is not a file descriptor useable to interact with
namespaces.

Cc: Jan Stancek <jstancek@redhat.com>
Cc: Cyril Hrubis <chrubis@suse.cz>
Link: https://lore.kernel.org/lkml/20200615085836.GR12456@shao2-debian
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>