gdb/{testsuite,system-gdbinit}: import gdb module in Python scripts

gdb/{testsuite,system-gdbinit}: import gdb module in Python scripts

Fix flake8 warnings like:

    gdb/system-gdbinit/wrs-linux.py:21:5: F821 undefined name 'gdb'

These scripts get executed in a context where the gdb module is already
loaded, so this is not strictly necessary.  However, adding these
imports removes a lot of red lines when editing these files in an IDE.
Without them, the code uses this `gdb` thing that appears to be
undefined.  Pylance is able to pull the module definition from typeshed
[1] and provide a good experience with typings.

[1] https://github.com/python/typeshed/tree/main/stubs/gdb/gdb

Change-Id: I09c6ae1866ef66f10d8270457771687343c84e32
Approved-By: Tom Tromey <tom@tromey.com>