Update UserNamespaceInKernel test requirement to handle redhat

On redhat based distribution, checking that USER_NS is compiled in the
kernel is not sufficient, we also have to check that the feature as
been enabled.

With this commit, it is now done by checking the content of
`/sys/module/user_namespace/parameters/enable`.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>