Method StringHolder

extlibs/catch/include/catch/catch.hpp:4755–4755

            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 );