Merge group test settings specified in Grunt config

Noticed a small issue the other day after the release of 0.2.4 where settings overrides for a group specified in grunt config weren't making it to tests. Specifically, I was overriding "globals" for an environment, but these weren't available via browser.globals for my environment test run, e.g.:

```
      options: {
        "settings": {
            ...
          }
        },
        "test_settings": {
          "default": {
            "globals": {
              "foo": "value1"
            }
          },
          "other_env": {
            "globals": {
              "foo": "value2"
            },
          },
```

With the above config in my Gruntfile, I'd end up with a browser.globals of {}. Patch below merges test_settings for a selected group into the big settings hash. 

Would appreciate a sanity check here, just in case I'm overlooking something about how settings from other sources (e.g. nightwatch.json) are merged.