Use `to_reference` in reference filter specs

...lib/gitlab/markdown/commit_range_reference_filter_spec.rb

spec/lib/gitlab/markdown/commit_reference_filter_spec.rb

...b/gitlab/markdown/external_issue_reference_filter_spec.rb

spec/lib/gitlab/markdown/issue_reference_filter_spec.rb

spec/lib/gitlab/markdown/label_reference_filter_spec.rb

...ib/gitlab/markdown/merge_request_reference_filter_spec.rb

spec/lib/gitlab/markdown/snippet_reference_filter_spec.rb

spec/lib/gitlab/markdown/user_reference_filter_spec.rb

spec/support/reference_filter_spec_helper.rb