Method StringHolder

tutorials/external/catch.hpp:4362–4362

            struct StringHolder : MatcherBase<NSString*>{
                StringHolder( NSString* substr ) : m_substr( [substr copy] ){}
                StringHolder( StringHolder const& other ) : m_substr( [other.m_substr copy] ){}
                StringHolder() {
                    arcSafeRelease( m_substr );