clang-tidy: fix some warnings

Resolve the following:
```
sys_info_param.cpp:12:11: style: Variable 'callback' can be declared as reference to const [constVariableReference]
    auto& callback = iterator->second;
          ^
include/ipmid/sessionhelper.hpp:49:31: performance: Ineffective call of function 'substr' because a prefix of the string is assigned to itself. Use resize() or pop_back() instead. [uselessCallsSubstr]
            sessionIdString = sessionIdString.substr(0, pos);
                              ^
include/ipmid/sessionhelper.hpp:36:14: style: The scope of the variable 'tempSessionHandle' can be reduced. [variableScope]
    uint16_t tempSessionHandle = 0;
             ^
include/ipmid/sessionhelper.hpp:77:47: performance: Function parameter 'objectPath' should be passed by const reference. [passedByValue]
bool isSessionObjectMatched(const std::string objectPath,
                                              ^
include/ipmid/sessionhelper.hpp:50:31: performance: Ineffective call of function 'substr' because a prefix of the string is assigned to itself. Use resize() or pop_back() instead. [uselessCallsSubstr]
            sessionIdString = sessionIdString.substr(0, pos);
                              ^
transport/serialbridge/test/serial_unittest.cpp:21:13: style: The scope of the variable 'c' can be reduced. [variableScope]
    uint8_t c;
            ^
```

Change-Id: I5a34fc50a95c33367e8b5d67dcb94c46b8def94a
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/transport/serialbridge/test/serial_unittest.cpp b/transport/serialbridge/test/serial_unittest.cpp
index 3fb0227..a7e2938 100644
--- a/transport/serialbridge/test/serial_unittest.cpp
+++ b/transport/serialbridge/test/serial_unittest.cpp
@@ -18,12 +18,11 @@
 
 TEST(TestSpecialCharact, getUnescapedCharact)
 {
-    uint8_t c;
     auto channel = std::make_shared<SerialChannel>(0);
 
     for (const auto& set : testsets)
     {
-        c = channel->getUnescapedCharacter(set.second);
+        uint8_t c = channel->getUnescapedCharacter(set.second);
         ASSERT_EQ(c, set.first);
     }
 }