Fixed arcs.t to work with new serialization mechanism

lib/Slic3r/ExtrusionPath.pm

@@ -148,6 +148,7 @@ sub detect_arcs {
     my $self = shift;
     my ($max_angle, $len_epsilon) = @_;
     
+    $self->deserialize;
     $max_angle = deg2rad($max_angle || 15);
     $len_epsilon ||= 10 / $Slic3r::scaling_factor;