platform/tests: fix bug in netns test which can cause mounting tmpfs over /run

platform/tests: fix bug in netns test which can cause mounting tmpfs over /run

The root-test "/general/netns/bind-to-path" mounts a tmpfs
over /var/run for the duration of the test and unmounts it at
the end.

Due to a bug, we first mount the tmpfs, then switch to another
namespace, and finally unmount /var/run on that other namespace.

That means, when you run the tests as root, it will bork /run
on your system.

Fixes: 46f5c07643e6cbf920a44bebe9ac9d3435a5a7b6