Add test for another edge case in PrivateNamingConvention

I was trying out this new rule in a few more places, and noticed
similarly broken behavior when the file starts with an @import
statement. My previous fix also resolves this, but I think it is good to
cover this with a test anyway.

spec/scss_lint/linter/private_naming_convention_spec.rb

@@ -60,6 +60,20 @@
       it { should_not report_lint }
     end
 
+    context 'is defined and used in the same file that starts with an @import' do
+      let(:scss) { <<-SCSS }
+        @import 'bar';
+
+        $_foo: red;
+
+        p {
+          color: $_foo;
+        }
+      SCSS
+
+      it { should_not report_lint }
+    end
+
     context 'is defined and used in a for loop when the file begins with a comment' do
       let(:scss) { <<-SCSS }
         // A comment