fix emscripten dependencies by making separate test crate

lib/emscripten-tests/emtests/test_exceptions_std.cpp

@@ -0,0 +1,22 @@
+// 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 <stdio.h>
+#include <exception>
+
+int main() {
+  std::exception e;
+  try {
+    throw e;
+  }
+  catch (std::exception e) {
+    printf("what? %s\n", e.what());
+    printf("caught std::exception\n");
+  }
+  catch (int x) {
+    throw std::bad_exception();
+  }
+  return 0;
+}