Avoid confusing CI reporting (#4953)

If the tests fails before starting the server (for instance while running eslint), the step debugging the server output was producing weird errors due to output files not being found.

.github/workflows/testing.yml

@@ -45,6 +45,7 @@ jobs:
         run: yarn test:client
 
       - name: Build and start server
+        id: server
         env:
           ENV_FILE: testing/.env
         run: |
@@ -79,7 +80,7 @@ jobs:
           diff -s testing/content/files/en-us/markdown/tool/h2m/index.md testing/content/files/en-us/markdown/tool/h2m/expected.md
 
       - name: Debug server's stdout and stderr if tests failed
-        if: failure()
+        if: failure() && steps.server.outcome != 'skipped'
         run: |
           echo "STDOUT..................................................."
           cat /tmp/stdout.log