Add additional coverage tests.

src/ftxui/dom/spinner_test.cpp

@@ -0,0 +1,42 @@
+#include <gtest/gtest-message.h>  // for Message
+#include <gtest/gtest-test-part.h>  // for SuiteApiResolver, TestFactoryImpl, TestPartResult
+#include "gtest/gtest_pred_impl.h"       // for Test, EXPECT_EQ, TEST
+#include <string>                   // for allocator
+#include "ftxui/dom/elements.hpp"        // for text, flexbox
+#include "ftxui/screen/screen.hpp"       // for Screen
+
+namespace ftxui {
+
+TEST(SpinnerTest, Spinner1) {
+  auto element = spinner(1, 0);
+  Screen screen(4, 1);
+  Render(screen, element);
+  EXPECT_EQ(screen.ToString(), ".   ");
+}
+
+TEST(SpinnerTest, Spinner2) {
+  auto element = spinner(1, 1);
+  Screen screen(4, 1);
+  Render(screen, element);
+  EXPECT_EQ(screen.ToString(), "..  ");
+}
+
+TEST(SpinnerTest, Spinner3) {
+  auto element = spinner(1, 2);
+  Screen screen(4, 1);
+  Render(screen, element);
+  EXPECT_EQ(screen.ToString(), "... ");
+}
+
+TEST(SpinnerTest, Spinner4) {
+  auto element = spinner(1, 3);
+  Screen screen(4, 1);
+  Render(screen, element);
+  EXPECT_EQ(screen.ToString(), ".   ");
+}
+
+} // namespace ftxui
+
+// Copyright 2022 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.