Substitution (when turned on via -s) will substitute into program strings

Test-Case: test_subst_bug2.py
## Description:

That is not the ideal behavior (ideal being defined by (1) what I
would expect in my program, and (2) what the C preprocessor does).

Start with the mentioned test case, test_subst_bug2.py, and inspect how substitution behaves inside program strings when -s is enabled. Compare the observed behavior with the expected C preprocessor behavior described in the issue; done means the test captures and passes the corrected behavior.

Written by the indexing model from the issue text.