[gdb/testsuite] Fix gdb.ada/mi_task_arg.exp on arm-linux

On arm-linux, I run into:
...
PASS: gdb.ada/mi_task_arg.exp: mi runto task_switch.break_me
Expecting: ^(-stack-list-arguments 1[^M
]+)?(\^done,stack-args=\[frame={level="0",args=\[\]},frame={level="1",args=\[{name="<_task>",value="0x[0-9A-Fa-f]+"}(,{name="<_taskL>",value="[0-9]+"})?\]},frame={level="2",args=\[({name="self_id",value="(0x[0-9A-Fa-f]+|<optimized out>)"})?\]},.*[^M
]+[(]gdb[)] ^M
[ ]*)
-stack-list-arguments 1^M
^done,stack-args=[frame={level="0",args=[]},frame={level="1",args=[{name="<_task>",value="0x40bc48"}]},frame={level="2",args=[]}]^M
(gdb) ^M
FAIL: gdb.ada/mi_task_arg.exp: -stack-list-arguments 1 (unexpected output)
...

The problem is that the test-case expects a level 3 frame, but there is none.

This can be reproduced using cli bt:
...
 $ gdb -q -batch outputs/gdb.ada/mi_task_arg/task_switch \
   -ex "b task_switch.break_me" \
   -ex run \
   -ex bt
 Breakpoint 1 at 0x34b4: file task_switch.adb, line 57.

 Thread 3 "my_caller" hit Breakpoint 1, task_switch.break_me () \
   at task_switch.adb:57
 57	      null;
 #0  task_switch.break_me () at task_switch.adb:57
 #1  0x00403424 in task_switch.caller (<_task>=0x40bc48) at task_switch.adb:51
 #2  0xf7f95a08 in ?? () from /lib/arm-linux-gnueabihf/libgnarl-12.so
 Backtrace stopped: previous frame identical to this frame (corrupt stack?)
...

The purpose of the test-case is printing the frame at level 1, so I don't
think we should bother about the presence of the frame at level 3.

Fix this by allowing the backtrace to stop at level 2.

Tested on arm-linux.

Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>