FIX Add test cases for SortPlugin

[sabina-talipova]

Parent issue

• SortPlugin: Add PHP Unit tests  #565

[GuySartorelli]

You have only one test validating that this sort plugin works when sorting multiple fields at once:
files(sort: { name: ASC, title: DESC }) {

Please also add one with those fields reversed, to validate that the order in which the fields are added in the query changes the actual sort order.
files(sort: { title: DESC, name: ASC }) {
Make sure this new scenario gives different results than the existing one.

[GuySartorelli]

Since this is now a bugfix, please update the commit message to use the FIX prefix and to have a message that explains what bug is being fixed.

[GuySartorelli]

Just one small change - everything else looks good.

[GuySartorelli]

LGTM