Fix CPPCheck issues - variableScope - Reduce variable scope

[eljonny]

Introduced by #27 while working on #8.

include/internal/TestCPPAssertions.h:304:42: style: The scope of the variable 'nullAssertionMessage' can be reduced. [variableScope]
            static constexpr const char* nullAssertionMessage = "Null assertion failed!";
                                         ^
include/internal/TestCPPAssertions.h:324:42: style: The scope of the variable 'notNullAssertionMessage' can be reduced. [variableScope]
            static constexpr const char* notNullAssertionMessage = "Not Null assertion failed!";
                                         ^