Added first unit test for bridge direction

t/bridges.t

@@ -0,0 +1,34 @@
+use Test::More tests => 1;
+use strict;
+use warnings;
+
+BEGIN {
+    use FindBin;
+    use lib "$FindBin::Bin/../lib";
+}
+
+use List::Util qw(first);
+use Slic3r;
+use Slic3r::Geometry qw(scale);
+use Slic3r::Test;
+
+my $flow = Slic3r::Flow->new(width => 0.5, spacing => 0.45, nozzle_diameter => 0.5);
+
+{
+    my $lower = Slic3r::ExPolygon->new(
+        Slic3r::Polygon->new_scale([0,0], [20,0], [20,10], [0,10]),
+        Slic3r::Polygon->new_scale([2,2], [2,8], [18,8], [18,2]),
+    );
+    my $bridge = $lower->[1]->clone;
+    $bridge->reverse;
+    $bridge = Slic3r::ExPolygon->new($bridge);
+    my $bd = Slic3r::Layer::BridgeDetector->new(
+        lower_slices    => [$lower],
+        perimeter_flow  => $flow,
+        infill_flow     => $flow,
+    );
+    # 0 is North/South
+    is $bd->detect_angle($bridge), 0, 'correct bridge angle detected';
+}
+
+__END__