dbus-daemon test: Don't test fd limits if in an unprivileged container

dbus-daemon test: Don't test fd limits if in an unprivileged container

In an unprivileged container, uid 0 doesn't have CAP_SYS_RESOURCE, so
we can't expect the dbus-daemon to be able to escalate its fd limit.

This can be reproduced using bubblewrap:

    sudo bwrap \
        --cap-drop CAP_SYS_RESOURCE \
        --ro-bind / / \
        --dev /dev \
    env \
        DBUS_TEST_DAEMON=.../bus/dbus-daemon \
        DBUS_TEST_DATA=.../test/data \
    .../test/test-dbus-daemon \
        -p /fd-limit \
        --verbose

Bug-Debian: https://bugs.debian.org/908092