Fix building tests with GCC-6

[Peter-Levine]

Building tests with GCC-6.3.0 fails with:

/var/tmp/portage/dev-python/shiboken-1.2.2/work/shiboken-1.2.2/tests/libsample/simplefile.cpp: In member function 'bool SimpleFile::exists() const':
/var/tmp/portage/dev-python/shiboken-1.2.2/work/shiboken-1.2.2/tests/libsample/simplefile.cpp:93:12: error: cannot convert 'std::ifstream {aka std::basic_ifstream<char>}' to 'bool' in return
     return ifile;
            ^~~~~
/var/tmp/portage/dev-python/shiboken-1.2.2/work/shiboken-1.2.2/tests/libsample/simplefile.cpp: In static member function 'static bool SimpleFile::exists(const char*)':
/var/tmp/portage/dev-python/shiboken-1.2.2/work/shiboken-1.2.2/tests/libsample/simplefile.cpp:100:12: error: cannot convert 'std::ifstream {aka std::basic_ifstream<char>}' to 'bool' in return
     return ifile;
            ^~~~~

Post C++11, implicit conversion to bool from an istream of ostream object is not allowed.