Add add_test_dependency method to Specification

While updating a gem of ours, we needed to add gems to the test group for bundler, but that functionality does not exist in the gemspec. So we added it the Gemfile but we get the following warnings while running `bundle install`:
```
Your Gemfile lists the gem rake (>= 0) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
Your Gemfile lists the gem rspec (>= 0) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
```
We added the `add_test_dependency` method (with tests) to make it possible to add test-related gems to the gemspec. 

How do we test this pull request with bundler locally?

Thanks, 
Alex and Roy