Add "new-ui console" tests

Add "new-ui console" tests

This adds a test that uses new-ui to create a secondary console, and
then runs some basic smoke tests.  It ensures that:

 - synchronous commands send output to the UI that initiated it

 - asynchronous events like breakpoint hits are reported on all
   consoles.

 - "new-ui" without arguments doesn't crash.

 - The "new-ui" command doesn't repeat.

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

	* gdb.base/new-ui.exp: New file.
	* lib/mi-support.exp (switch_gdb_spawn_id): Move to ...
	* lib/gdb.exp (switch_gdb_spawn_id): ... here.
	(with_spawn_id): New procedure.