Improve testcases for unsigned char platforms (#5524)

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)`.

Author: moui0

test/testcondition.cpp

@@ -4498,7 +4498,11 @@ class TestCondition : public TestFixture {
               "        if (o[1] == '\\0') {}\n"
               "    }\n"
               "}\n");
-        ASSERT_EQUALS("[test.cpp:6]: (style) Condition 'o[1]=='\\0'' is always false\n", errout.str());
+        if (std::numeric_limits<char>::is_signed) {
+            ASSERT_EQUALS("[test.cpp:6]: (style) Condition 'o[1]=='\\0'' is always false\n", errout.str());
+        } else {
+            ASSERT_EQUALS("[test.cpp:4] -> [test.cpp:6]: (style) Condition 'o[1]=='\\0'' is always false\n", errout.str());
+        }
 
         check("void f(int x) {\n" // #11449
               "    int i = x;\n"
@@ -5016,7 +5020,11 @@ class TestCondition : public TestFixture {
               "       buffer.back() == '\\n' ||\n"
               "       buffer.back() == '\\0') {}\n"
               "}\n");
-        ASSERT_EQUALS("[test.cpp:5]: (style) Condition 'buffer.back()=='\\0'' is always false\n", errout.str());
+        if (std::numeric_limits<char>::is_signed) {
+            ASSERT_EQUALS("[test.cpp:5]: (style) Condition 'buffer.back()=='\\0'' is always false\n", errout.str());
+        } else {
+            ASSERT_EQUALS("[test.cpp:3] -> [test.cpp:5]: (style) Condition 'buffer.back()=='\\0'' is always false\n", errout.str());
+        }
 
         // #9353
         check("typedef struct { std::string s; } X;\n"