Make mi-break.exp always expect breakpoint commands output on the main UI

Make mi-break.exp always expect breakpoint commands output on the main UI

mi-break.exp regresses when tested with MI running on a secondary UI,
with RUNTESTFLAGS="FORCE_SEPARATE_MI_TTY=1".

The problem is simply that the test sets a breakpoint, and attaches
"print" commands to the breakpoint.  Since breakpoint commands always
run with the main UI as current UI, the breakpoint command's output
goes to the main UI.  So we need to tweak the test to expect it there.

gdb/testsuite/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* gdb.mi/mi-break.exp (test_breakpoint_commands): Always expect
	breakpoint command's output on the main UI.
	(test_break): New procedure, factored out from calls in the top
	level.
	(top level): Use foreach_with_prefix to test MI as main UI and as
	separate UI.