EXPECT_FALSE(absl::StrContains(s, expected))
      << "'" << s << "' should not contain '" << expected << "'";
}

// WritableFile that simply concats into string.
class StringWritableFile : public WritableFile {
 public:
  explicit StringWritableFile(string* str) : str_(*str) {}

  Status Append(StringPiece data) override {
    absl::StrAppend(&str_, data);
    return absl::OkStatus();
  }