fix emscripten dependencies by making separate test crate

lib/emscripten-tests/emtests/test_exceptions_libcxx.cpp

@@ -0,0 +1,20 @@
+// Copyright 2016 The Emscripten Authors.  All rights reserved.
+// Emscripten is available under two separate licenses, the MIT license and the
+// University of Illinois/NCSA Open Source License.  Both these licenses can be
+// found in the LICENSE file.
+
+#include <string>
+#include <iostream>
+#include <stdexcept>
+
+int main() {
+  std::string s;
+  try {
+    std::cout << s.at(0) << std::endl;
+  } catch (const std::out_of_range& e) {
+    std::cout << "Exception caught:" << std::endl;
+    std::cout << e.what() << std::endl;
+  }
+  return 0;
+}
+