gdb/testsuite/tui: Introduce check_box_contents

gdb/testsuite/tui: Introduce check_box_contents

A new test procedure for matching the contents of one screen box
against a regexp.  This can be used to match the contents of one TUI
window against a regexp without any of the borders, or other windows
being included in the matched output (as is currently the case with
check_contents).

This will be used in a later commit.

gdb/testsuite/ChangeLog:

	* lib/tuiterm.exp (Term::check_box_contents): New proc.

Change-Id: Icf795bf38dd9295e282a34eecc318a9cdbc73926