Make `@import` paths relative to `./stylesheets`

The `@import` paths at the top of `spec/stylesheets/test.scss` were relative to the root of the repository. Because the Load path is set to 'root + `./stylesheets`' instead, this was breaking the tests.

For example:

    @import 'stylesheets/_colours.scss';

...resolves to:

    @import 'govuk_frontend_toolkit/stylesheets/stylesheets/_colours.scss';

This changes the import paths at the top of `spec/stylesheets/test.scss` to be relative to `govuk_frontend_toolkit/stylesheets` which matches the Load path.
This line is a tiny bit intense. Could it be split out a little so it is more parseable. Something like

```javascript
if(typeof subdir !== 'undefined'){
  var relpath = subdir + '/' + filename;
} else {
  var relpath = filename;
}
```

while more verbose is easier to follow what is being concatenated to what.