Method StringHolder

Bcore/src/main/cpp/Dobby/tests/catch.hpp:2811–2812

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