Force build before test to avoid false positives

If a commit eg: f916185 doesn't build the riot.js file it may pass CI although it fails the tests after building. This should help that situation.

This is the failing test I'm seeing:

```
--- $.render ---
Single token
Multiple tokens
Single quotes
Empty value

AssertionError: "null" != ""
...
at ... riotjs/test/render_test.js:21:12
```


what is change for?