Round-trip the zwj unicode character

Rendering of sequences containing zwj is still not implemented, since it
can cause the collapse of an unbounded number of characters into a
single cell. However, kitty at least preserves the zwj by storing it as
a combining character.

kitty_tests/screen.py

@@ -96,6 +96,13 @@ class TestScreen(BaseTest):
         self.ae(str(s.line(0)), q)
         self.ae(s.cursor.x, 2)
 
+    def test_zwj(self):
+        s = self.create_screen(cols=20)
+        q = '\U0001f468\u200d\U0001f469\u200d\U0001f467\u200d\U0001f466'
+        s.draw(q)
+        self.ae(q, str(s.line(0)))
+        self.ae(s.cursor.x, 8)
+
     def test_char_manipulation(self):
         s = self.create_screen()