Added Tests. Added Contains method

FlexArray.Test/FlexArrayStringTests.cs

@@ -83,4 +83,51 @@ public class FlexArrayStringTests
         array.Capacity.Should().BeGreaterThan(FlexArrayString.DefaultStartSize, "capacity exceeded, had to grow"); 
         array.Count.Should().Be(FlexArrayString.DefaultStartSize + 2);
     }
+    
+    // Contains tests
+    [Fact]
+    public void Contains_Success()
+    {
+        var array = new FlexArrayString();
+        array.Add("Goodbye");
+        array.Add("World");
+        array.Add("!");
+        array.Add("bye...");
+
+        array.Contains("!")
+             .Should().BeTrue();
+    }
+
+    [Fact]
+    public void Contains_Success_AfterGrowth()
+    {
+        var array = new FlexArrayString();
+        for (var i = 0; i <= FlexArrayString.DefaultStartSize; i++)
+        {
+            array.Add(i.ToString());
+        }
+
+        array.Contains(array[FlexArrayString.DefaultStartSize - 1])
+             .Should().BeTrue();
+    }
+
+    [Fact]
+    public void Contains_Empty()
+    {
+        var array = new FlexArrayString();
+
+        array.Contains("Hi!")
+             .Should().BeFalse("collection is empty");
+    }
+
+    [Fact]
+    public void Contains_NotFound()
+    {
+        var array = new FlexArrayString();
+        array.Add("Hello");
+        array.Add("World");
+
+        array.Contains("!")
+             .Should().BeFalse("not contained in list");
+    }
 }