Added visitor pattern test case

docs/test_cases/t00021.md

@@ -0,0 +1,74 @@
+# t00021 - Visitor pattern
+## Config
+```yaml
+compilation_database_dir: ..
+output_directory: puml
+diagrams:
+  t00021_class:
+    type: class
+    glob:
+      - ../../tests/t00021/t00021.cc
+    using_namespace:
+      - clanguml::t00021
+    include:
+      namespaces:
+        - clanguml::t00021
+
+```
+## Source code
+File t00021.cc
+```cpp
+#include <memory>
+
+namespace clanguml {
+namespace t00021 {
+
+class A;
+class B;
+
+class Visitor {
+public:
+    virtual ~Visitor() = default;
+    virtual void visit_A(const A &item) const = 0;
+    virtual void visit_B(const B &item) const = 0;
+};
+
+class Visitor1 : public Visitor {
+public:
+    void visit_A(const A &item) const override { }
+    void visit_B(const B &item) const override { }
+};
+
+class Visitor2 : public Visitor {
+public:
+    void visit_A(const A &item) const override { }
+    void visit_B(const B &item) const override { }
+};
+
+class Visitor3 : public Visitor {
+public:
+    void visit_A(const A &item) const override { }
+    void visit_B(const B &item) const override { }
+};
+
+class Item {
+public:
+    virtual ~Item() = default;
+    virtual void accept(const Visitor &visitor) const = 0;
+};
+
+class A : public Item {
+public:
+    void accept(const Visitor &visitor) const override { }
+};
+
+class B : public Item {
+public:
+    void accept(const Visitor &visitor) const override { }
+};
+}
+}
+
+```
+## Generated UML diagrams
+![t00021_class](./t00021_class.png "Visitor pattern")