gdb/testsuite: use "not in" in Python files

Fix flake8 warnings like this one:

    gdb/testsuite/gdb.python/py-send-packet.py:68:12: E713 test for membership should be 'not in'

Change-Id: I2b4cc1eeb63ee2fceb8c4264e7d6ce2d22824688
Approved-By: Tom Tromey <tom@tromey.com>
2 files changed