Added is-in-rex and not-in-rex tests

Regular expression based is-in and not-in tests added.
This permits tests to be performed based on ics:regex e.g.

iterate /interfaces/interface[name="vlan"]/unit {
    is-in-rex name,"^1[02468]0[0-3]$","^2[02345]0[0-3]$" {
        info Checking VLANs;
        err "Additional VLANs found %s",name;
    }
}

iterate /interfaces/interface[name="vlan"]/unit {
    not-in-rex name,"^1500$","^2500$" {
        info Check VLANs 1500 and 2500 are not defined;
        err "Found VLAN %s",name;
    }       
}