Add code coverage support. (#378)

src/ftxui/dom/text_test.cpp

@@ -8,7 +8,7 @@
 #include "ftxui/screen/screen.hpp"  // for Screen
 #include "gtest/gtest_pred_impl.h"  // for Test, EXPECT_EQ, TEST
 
-using namespace ftxui;
+namespace ftxui {
 
 TEST(TextTest, ScreenHeightSmaller) {
   auto element = text("test");
@@ -103,6 +103,8 @@ TEST(TextTest, CombiningCharacters) {
   EXPECT_EQ(t, screen.ToString());
 }
 
+}  // namespace ftxui
+
 // Copyright 2020 Arthur Sonzogni. All rights reserved.
 // Use of this source code is governed by the MIT license that can be found in
 // the LICENSE file.