Added a couple unit tests

t/fill.t

@@ -0,0 +1,30 @@
+use Test::More;
+use strict;
+use warnings;
+
+plan tests => 2;
+
+BEGIN {
+    use FindBin;
+    use lib "$FindBin::Bin/../lib";
+}
+
+use Slic3r;
+
+my $print = Slic3r::Print->new(
+    x_length => 50,
+    y_length => 50,
+);
+
+{
+    my $filler = Slic3r::Fill::Rectilinear->new(print => $print);
+    my $surface_width = 250;
+    my $distance = $filler->adjust_solid_spacing(
+        width       => $surface_width,
+        distance    => 100,
+    );
+    is $distance, 125, 'adjusted solid distance';
+    is $surface_width % $distance, 0, 'adjusted solid distance';
+}
+
+__END__