Problem: No test for closing tab by click in tabline.
Solution: Add a test. Also fix that dragging window separator could fail in
a large terminal. (Dominique Pelle, closes#4253)
Problem: No test for mouse clicks in the terminal tabpage line.
Solution: Add a test. (Dominique Pelle, closes#4247). Also init
TabPageIdxs[], in case it's used before a redraw.
Problem: Termcodes test would fail in a very big terminal.
Solution: Bail out when the row is larger than what will work. (Dominique
Pelle, closes#4246)
Problem: Termcodes tests can be improved.
Solution: Add helper functions to simplify tests. Dragging statusline for
xterm and sgr. (Dominique Pelle, closes#4237)
Problem: Decoding of mouse click escape sequence is not tested.
Solution: Add a test for xterm and SGR using low-level input. Make
low-level input execution with feedkeys() work.
