[gdb/testsuite] Fix gdb.python/py-mi-cmd.exp with python 3.13

When running test-case gdb.python/py-mi-cmd.exp with python 3.13, I run into:
...
Expecting: ^(-pycmd exp[^M
]+)?(.*&"Traceback \(most recent call last\):.."^M
&"[^^M
]+py-mi-cmd.py[^^M
]+"^M
&"[^^M
]+raise gdb.GdbError\(\).."^M
&"gdb.GdbError.."^M
\^error,msg="Error occurred in Python\."[^M
]+[(]gdb[)] ^M
[ ]*)
-pycmd exp^M
&"Traceback (most recent call last):\n"^M
&"  File \"py-mi-cmd.py\", line 76, in invoke\n    raise gdb.GdbError()\n"^M
&"gdb.GdbError\n"^M
^error,msg="Error occurred in Python."^M
(gdb) ^M
FAIL: gdb.python/py-mi-cmd.exp: -pycmd exp (unexpected output)
...

In contrast, with python 3.12 I have:
...
Expecting: ^(-pycmd exp[^M
]+)?(.*&"Traceback \(most recent call last\):.."^M
&"[^^M
]+py-mi-cmd.py[^^M
]+"^M
&"[^^M
]+raise gdb.GdbError\(\).."^M
&"gdb.GdbError.."^M
\^error,msg="Error occurred in Python\."[^M
]+[(]gdb[)] ^M
[ ]*)
-pycmd exp^M
&"Traceback (most recent call last):\n"^M
&"  File \"py-mi-cmd.py\", line 76, in invoke\n"^M
&"    raise gdb.GdbError()\n"^M
&"gdb.GdbError\n"^M
^error,msg="Error occurred in Python."^M
(gdb) ^M
PASS: gdb.python/py-mi-cmd.exp: -pycmd exp
...

To make it easier to understand what we're looking at, let's take this out of
the mi interpreter context and use the cli interpreter:
...
$ gdb -q -batch -ex "set trace-commands on" -x gdb.in
+set python print-stack full
+source py-mi-cmd.py
+python pycmd1('-pycmd')
+python pycmd1.invoke (pycmd1, ["exp"])
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "py-mi-cmd.py", line 76, in invoke
    raise gdb.GdbError()
gdb.GdbError
gdb.in:4: Error in sourced command file:
Error occurred in Python.
...

Interestingly, this is what we're seeing with both python 3.12 and 3.13.

The difference between the python versions is that:
- with python 3.12 each line is printed by itself, and
- with python 3.13 two particular lines are printed toghether.

With the cli interpreter, that makes no difference, because the '\n' is
interpreted.

But with the mi interpreter, that causes a difference in output because the
'\n' is not interpreted, but rather printed literally.

Fix this by accepting the new output in addition to the old one.

Tested on aarch64-linux.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>

PR testsuite/31913
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31913