Function prepareExpandedExpression

Bcore/src/main/cpp/Dobby/tests/catch.hpp:11158–11160

namespace Catch {
void prepareExpandedExpression(AssertionResult &result) {
  result.getExpandedExpression();
}

// Because formatting using c++ streams is stateful, drop down to C is required
// Alternatively we could use stringstream, but its performance is... not good.