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.