gdb/testsuite introduce foreach_mi_ui_mode helper proc

gdb/testsuite introduce foreach_mi_ui_mode helper proc

Introduce foreach_mi_ui_mode, a helper proc which can be used when
tests are going to be repeated once with the MI in the main UI, and
once with the MI on a separate UI.

The proc is used like this:

  foreach_mi_ui_mode VAR {
    # BODY
  }

The BODY will be run twice, once with VAR set to 'main' and once with
VAR set to 'separate', inside BODY we can then change the behaviour
based on the current UI mode.

The point of this proc is that we sometimes shouldn't run the separate
UI tests (when gdb_debug_enabled is true), and this proc hides all
this logic.  If the separate UI mode should not be used then BODY will
be run just once with VAR set to 'main'.

I've updated two tests that can make use of this helper proc.  I'm
going to add another similar test in a later commit.

There should be no change to what is tested with this commit.

Approved-By: Pedro Alves <pedro@palves.net>