test: skip test_exec_networknamespacepath if netns setup fails

test: skip test_exec_networknamespacepath if netns setup fails

In some environments, such as a LXD container, the netns setup might
fail because ip netns exec fails trying to mount /sys:

 $ systemd-detect-virt
 lxc
 $ ip link add dummy-test-exec type dummy
 $ ip netns add test-execute-netns
 $ ip netns exec test-execute-netns ip link add dummy-test-ns type dummy
 mount of /sys failed: Operation not permitted

If this setup fails, test_exec_networknamespacepath will fail, so check
the exit codes for these setup calls and skip the test if necessary.