Improve testcases for unsigned char platforms

[moui0]

I got error messages while building cppcheck 2.12.0 for RISC-V Arch Linux:

Testing Complete
Number of tests: 4420
Number of todos: 331
Tests failed: 2

/usr/src/debug/cppcheck/cppcheck/test/testcondition.cpp:4501(TestCondition::alwaysTrue): Assertion failed. 
Expected: 
[test.cpp:6]: (style) Condition 'o[1]=='\0'' is always false\n

Actual: 
[test.cpp:4] -> [test.cpp:6]: (style) Condition 'o[1]=='\0'' is always false\n

_____
/usr/src/debug/cppcheck/cppcheck/test/testcondition.cpp:5014(TestCondition::alwaysTrueContainer): Assertion failed. 
Expected: 
[test.cpp:5]: (style) Condition 'buffer.back()=='\0'' is always false\n

Actual: 
[test.cpp:3] -> [test.cpp:5]: (style) Condition 'buffer.back()=='\0'' is always false\n

I found out the reason is that the testcases were designed for x86/x86_64 or other signed char platforms (i.e. default character type is signed char ), whereareas RISC-V is an unsigned char platform, which causes different behavior in lib/valueflow.cpp:valueFlowImpossibleValues. I'm not sure whether this error leads from a functional bug, so if you have a better approach to fix it, please let me know.

Maybe you could reproduce this error on x86_64 platform by setting defaultSign = 'u'; in Platform::set(Type t).

[chrchr-github]

See https://trac.cppcheck.net/ticket/11537

[moui0]

Is it possible to fix this problem by modifying valueflow.cpp, as the current fix is not efficient?

Perhaps a similar code block can be added for signed, but this may lead to similar issues in many testcases on the x86 platform:

https://github.com/danmar/cppcheck/blob/723ba16eb657b4369d3d67727ae271bec6d8bfad/lib/valueflow.cpp#L1836-L1844

[chrchr-github]

@moui0 How do you want to be credited as an author?

[moui0]

Wang Haoyu, please. Thank you!