Unit test - Environment::find 

While writing some docs, I noticed the [Environment](https://github.com/RcppCore/Rcpp/blob/6f81b4684481dbd9bb554dd95e66725fc3b63a8c/inst/unitTests/cpp/Environment.cpp) unit test setup is missing routines for `find`. Also, when I added the ability to use symbols with `find` and `get` in #326, I did not add a unit test... 