ci: cleanup Electron dirs before running tests (#14989)

vsts.yml

@@ -85,14 +85,16 @@ jobs:
       if pgrep Electron; then
         killall Electron
       fi
+      rm -rf ~/Library/Saved\ Application\ State/com.github.electron.savedState
+      rm -rf ~/Library/Application\ Support/Electron
     displayName: Make sure Electron isn't running from previous tests
-# FIXME(alexeykuzmin)
-#  - bash: |
-#      cd src
-#      python electron/script/verify-ffmpeg.py --source-root "$PWD" --build-dir out/Default --ffmpeg-path out/ffmpeg
-#    displayName: Verify non proprietary ffmpeg
-#    condition: and(succeeded(), eq(variables['RUN_TESTS'], '1'))
-#    timeoutInMinutes: 5
+
+  - bash: |
+      cd src
+      python electron/script/verify-ffmpeg.py --source-root "$PWD" --build-dir out/Default --ffmpeg-path out/ffmpeg
+    displayName: Verify non proprietary ffmpeg
+    condition: and(succeeded(), eq(variables['RUN_TESTS'], '1'))
+    timeoutInMinutes: 5
 
   - bash: |
       cd src