Terminal sequences in check_files are hard to decipher

[rillig]

I am new to the curses test suite and when I saw addch.chk for the first time, I could not make any sense of it. I had no idea where all these strange letters such as smsotrmsocup6 should mean. Later I saw that these sequences are defined in atf.terminfo.

To me, the output would be much easier to decipher if the escape sequences had some delimiters, such as <smso>t<rmso><cup6>. This would make it immediately obvious where the t from the test input appears in the output.