test:Added DOT highlights test.

Author: antononcube

xt/20-graphviz-dot-format.rakutest

@@ -55,5 +55,32 @@ END
     is $res.indices('label="here"').elems, 1, 'Expected number of "here" edge labels';
 }, 'Labeled directed graph as mixed';
 
+## 3
+subtest {
+    my @edges = '1' => '2', '2' => '1', '1' => '3', '3' => '2';
+    my %edge-labels = '1' => { '2' => 'here'};
+    my %highlight = Red => ( '1' => '2', );
+    my $res = Graph.new(@edges, :directed).dot(:%highlight);
+
+    my $dot = q:to/END/;
+digraph {
+bgcolor="none";
+node [style=filled, fixedsize=true, shape=circle, color="Black", fillcolor="SteelBlue", penwidth=1, fontsize=12, fontcolor="White", labelloc=c, width=0.3, height=0.3];
+edge [color="SteelBlue", penwidth=2, fontsize=10, fontcolor="Ivory", arrowsize=1];
+"1"; "2"; "3";
+"2" -> "1"
+"1" -> "3"
+"3" -> "2"
+edge [color="Red", penwidth=2, arrowsize=1];
+"1" -> "2"
+}
+END
+
+    is $res.indices('->').elems, 4, 'Expected number of edge specs';
+    is $res.indices('[dir=forward]').elems, 0, 'Expected number of directed edge specs';
+    is $res.indices('[dir=none]').elems, 0, 'Expected number of un-directed edge specs';
+    is $res.indices('color="Red"').elems, 1, 'Expected number of Red color specs';
+
+}, 'Directed graph with highlights';
 
 done-testing;