test: unskip icon flex container unit tests (#10683) (#10685)

Co-authored-by: Serhii Kulykov <iamkulykov@gmail.com>

Author: vaadin-bot

packages/icon/test/icon.test.js

@@ -477,8 +477,7 @@ describe('vaadin-icon', () => {
     });
   });
 
-  // TODO: Enable when unit tests are using the base theme
-  describe.skip('flex container', () => {
+  describe('flex container', () => {
     let container;
 
     beforeEach(async () => {