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

The Load path is set to

    `govuk_frontend_toolkit/stylesheets`

so having the `@import` paths relative to

    `govuk_frontend_toolkit`

is breaking the tests.
Merge pull request #130 from alphagov/fix-import-paths-for-sass-test

Make `@import` paths relative to `./stylesheets`
Add test for side-effect of getSelections method

getSelections calls markSelected with the
assumption that the selections property is set on
their shared instance.
Remove temporary `selections` variable

All changes to the `selections` property were cached
in a local `selections` variable to be copied over
after the each loop ends.

markSelected can be called inside the loop and
uses the `selections` property so changes to that
property need to be made in the same iteration
as it is called.