Add code coverage support. (#378)

src/ftxui/dom/flexbox_test.cpp

@@ -9,7 +9,7 @@
 #include "ftxui/screen/screen.hpp"       // for Screen
 #include "gtest/gtest_pred_impl.h"       // for Test, EXPECT_EQ, TEST
 
-using namespace ftxui;
+namespace ftxui {
 
 TEST(FlexboxTest, BasicRow) {
   auto root = flexbox(
@@ -433,6 +433,8 @@ TEST(FlexboxTest, GapY) {
             "       ");
 }
 
+}  // namespace ftxui
+
 // Copyright 2021 Arthur Sonzogni. All rights reserved.
 // Use of this source code is governed by the MIT license that can be found in
 // the LICENSE file.