execute IWYU and add some coverage tests.

src/ftxui/screen/string_test.cpp

@@ -120,7 +120,7 @@ TEST(StringTest, CellToGlyphIndex) {
   EXPECT_EQ(combining[2], 2);
 }
 
-}
+}  // 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.