tests: add test bottle for Sierra

Fixes:

```
  1) Error:
FormularyFactoryTest#test_factory_from_bottle:
Errno::ENOENT: No such file or directory - /usr/local/Library/Homebrew/test/bottles/testball_bottle-0.1.sierra.bottle.tar.gz
    /usr/local/Library/Homebrew/formulary.rb:98:in `realpath'
    /usr/local/Library/Homebrew/formulary.rb:98:in `realpath'
    /usr/local/Library/Homebrew/formulary.rb:98:in `initialize'
    /usr/local/Library/Homebrew/formulary.rb:274:in `new'
    /usr/local/Library/Homebrew/formulary.rb:274:in `loader_for'
    /usr/local/Library/Homebrew/formulary.rb:215:in `factory'
    /usr/local/Library/Homebrew/test/test_formulary.rb:79:in `test_factory_from_bottle'
```

Closes #358.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>