gdb/testsuite: fix failure in gdb.python/py-unwind.exp

gdb/testsuite: fix failure in gdb.python/py-unwind.exp

A potential test failure was introduced with commit:

  commit 6bf5f25bb150c0fbcb125e3ee466ba8f9680310b
  Date:   Wed Mar 8 16:11:30 2023 +0000

      gdb/python: make the gdb.unwinder.Unwinder class more robust

In this commit a new test was added, however the expected output
pattern varies depending on which Python version GDB is linked
against.

Older versions of Python result in output like this:

    (gdb) python global_test_unwinder.name = "foo"
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    AttributeError: can't set attribute
    Error while executing Python code.
    (gdb)

While more recent versions of Python give a similar, but slightly more
verbose error message, like this:

    (gdb) python global_test_unwinder.name = "foo"
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    AttributeError: can't set attribute 'name'
    Error while executing Python code.
    (gdb)

The test was only accepting the first version of the output.  This
commit extends the test pattern so that either version will be
accepted.