`M0-1-9`: Add test for side effects in a different translation unit

[rcseacord]

There should be a test where the side-effecting and no-side effecting functions are in a different translation unit:

codeql-coding-standards/cpp/autosar/test/rules/M0-1-9/test.cpp

Line 1 in 9a23aba

int may_have_side_effects();

What is the expected behavior in such as case?