[Tests] no-unused-modules: enable flat config test

Author: timmywil

tests/src/rules/no-unused-modules.js

@@ -1374,7 +1374,7 @@ describe('parser ignores prefixes like BOM and hashbang', () => {
   });
 });
 
-describe('supports flat eslint', { skip: !FlatRuleTester }, () => {
+(FlatRuleTester ? describe : describe.skip)('supports flat eslint', () => {
   const flatRuleTester = new FlatRuleTester();
   flatRuleTester.run('no-unused-modules', rule, {
     valid: [{