gdb: fix regression in gdb.xml/maint_print_struct.exp

gdb: fix regression in gdb.xml/maint_print_struct.exp

A regression in gdb.xml/maint_print_struct.exp was introduced with
commit:

  commit 81b86eced24f905545b58aa6c27478104c364976
  Date:   Fri Jan 6 09:30:40 2023 -0700

      Do not record a rejected target description

The test relied on an invalid target description being stored within
the tdesc_info of the current inferior, the above commit stopped this
behaviour.

Update the test to check that the invalid architecture is NOT stored,
and then check printing the target description directly from the
file.

Approved-By: Tom Tromey <tromey@adacore.com>