Under valgrind, we cannot create an NAcd instance.

    --10916-- WARNING: unhandled amd64-linux syscall: 321
    --10916-- You may be able to write your own handler.
    --10916-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
    --10916-- Nevertheless we consider this a bug.  Please report
    --10916-- it at http://valgrind.org/support/bug_reports.html.

This limitation already poses a problem, because running NetworkManager
under valgrind might fail. However, for tests it doesn't matter and we
can just skip them.