fix(TestCases): Fix CE about static const member

This is reported in coder3101#129.

See https://stackoverflow.com/questions/27258344/compiler-error-or-correct-behavior-for-static-const-member-variable-variadic-te

Because

> If a non-volatile const static data member is of integral or
> enumeration type, its declaration in the class definition can
> specify a brace-or-equal-initializer in which every
> initializer-clause that is an assignment expression is a constant
> expression (5.19). [...] The member shall still be defined in a
> namespace scope if it is odr-used (3.2) in the program and the
> namespace scope definition shall not contain an initializer.

another definition in the .cpp file with namespace is required.

This should be a better solution than coder3101#135.

src/TestCases.cpp

@@ -420,6 +420,8 @@ bool TestCase::isPass() const
     return true;
 }
 
+const int TestCases::MAX_NUMBER_OF_TESTCASES;
+
 TestCases::TestCases(MessageLogger *logger, QWidget *parent) : QWidget(parent), log(logger)
 {
     mainLayout = new QVBoxLayout(this);