import methods::setPackageName so test can be run easily with Rscript

Upgraded to `testthat 0.8.1` today and started getting this error when running tests with `Rscript` and `devtools::test`.  This didn't show up when using `testthat 0.7.1`, because `testthat 0.8.1` moved `methods` from `Depends:` to `Imports:`, but should probably be taken care of here.

```
$ $ Rscript -e "library(devtools); test('pkg')"
Testing ygdp
Loading required package: testthat
Loading ygdp
Error in create_ns_env(pkg) : could not find function "setPackageName"
Calls: test -> load_all -> create_ns_env
recover called non-interactively; frames dumped, use debugger() to view
```