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.
