Make rootless namespace unit test execution work
Make rootless namespace unit test execution work

We should target to get the unit tests back to a state where we can
execute them in a rootless way. Right now the system user would not have
a chance to access pid 1's /proc/1/ns directory which make the test
fail. This patch uses the current pid, which should be accessible by the
current user executing the tests.

Can be verified by:
```
> make testunit TESTFLAGS=--focus=SandboxManagedNamespaces
```

Signed-off-by: Sascha Grunert <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@saschagrunert.de>

kubernetes-incubator/cri-o,kubernetes-incubator/cri-o,kubernetes-incubator/ocid,nalind/cri-o,kubernetes-incubator/ocid,kubernetes-incubator/cri-o,nalind/cri-o,kubernetes-incubator/cri-o,kubernetes-incubator/ocid,nalind/cri-o