Added test documentation

Added error messages when needed
and display names to all methods.

Author: J-manLans

src/test/java/com/thealgorithms/datastructures/trees/GenericTreeTest.java

@@ -101,7 +101,7 @@ void testCreateInValidTree() {
     @Test
     @DisplayName("Gets the correct number of nodes in the tree")
     void testGettingCorrectSizeOfTree() {
-        Assertions.assertEquals(3, tree.size2call());
+        Assertions.assertEquals(9, tree.size2call());
     }
 
     @Test