docs: added CounterTest.razor

Author: egil

docs/samples/tests/razor/CounterTest.razor

@@ -0,0 +1,16 @@
+@inherits TestContext
+@code {
+  [Fact]
+  public void CounterShouldIncrementWhenClicked()
+  {
+    // Arrange: render the Counter.razor component
+    var cut = Render(@<Counter />);
+
+    // Act: find and click the <button> element to increment
+    // the counter in the <p> element
+    cut.Find("button").Click();
+
+    // Assert: first find the <p> element, then verify its content
+    cut.Find("p").MarkupMatches(@<p>Current count: 1</p>);
+  }
+}