Add basic Sass test to check our Sass compiles

Slightly crufty initial implementation. The SCSS imports are static,
rather than generating them at test time and so being resilient to
future additions. I think that approach would require writing a grunt
plugin, or switching to use gulp. Punted for now.

Tested by reverting 31b973d11c968a5db5a4e4de5a19508eac932333 and running
the tests. They failed with:

```
 > grunt
Running "sass:development" (sass) task
Syntax error: Positional arguments must come before keyword arguments.
        on line 42 of /Users/jabley/govuk/govuk_frontend_toolkit/stylesheets/_typography.scss
        from line 2 of /Users/jabley/govuk/govuk_frontend_toolkit/stylesheets/design-patterns/_alpha-beta.scss
        from line 6 of spec/stylesheets/test.scss
  Use --trace for backtrace.
Warning: Exited with error code 1 Use --force to continue.

Aborted due to warnings.
```