[LINTER] Add an exception for a cppcheck false positive

Summary:
There is a false positive out of bounds reported in the
processor_tests.cpp. The size of the accessed array is asserted just
before accessing the element so there is no possible issue, but some
combination of boost/cppcheck fail to determine this (e.g. ArchLinux).

Test Plan:
Edit processor_tests.cpp with some dummy changes, then run arc lint.
Before the patch cppcheck reports errors, after the patch arc lint is
happy.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9594