gdb/testsuite: add some logging in Term::_check_box

I was diagnosing some problem with a TUI test case, which lead me to
improve the logging of _check_box a bit.  It did help me, so I think it
would be nice to have it upstream.


	* lib/tuiterm.exp (Term) <_check_box>: Improve logging.

Change-Id: I887e83c02507d6c59c991e17f795c844ed63bacf
2 files changed