Adjust MI to $inferior_spawn_id

Adjust MI to $inferior_spawn_id

Rather than trying to determine where (which spawn id) the inferior
output comes out from, which depends on e.g., remote that supports
file i/o remote protocol extension, vs remote that sends inferior
output through a separate $inferior_spawn_id, vs native debugging,
which sends output through $gdb_spawn_id, vs native debugging with a
test that uses "separate-inferior-tty" (like mi-console.exp does),
always expect inferior output from both $inferior_spawn_id and
$gdb_spawn_id.

mi-console.exp itself already copes with different possible outputs in
a similar way:

 # Combine both outputs in a single pattern.
 set output "($semihosted_output|$native_output)"

Fixes:

 FAIL: gdb.mi/mi-console.exp: Testing console output inferior output (timeout)

when testing against local gdbserver with gdb,noinferiorio removed
from the board file.

gdb/testsuite/ChangeLog:
2015-07-29  Pedro Alves  <palves@redhat.com>

	* lib/mi-support.exp (mi_inferior_spawn_id): Delete.
	(default_mi_gdb_start): Set inferior_spawn_id instead of
	mi_inferior_spawn_id.  If $inferior_spawn_id is not set, set it to
	gdb_spawn_id.
	(mi_gdb_test): Always expect inferior output from both
	$inferior_spawn_id and $gdb_spawn_id.