[java] Start Firefox headless in server tests

Author: barancev

.github/workflows/java.yml

@@ -174,6 +174,8 @@ jobs:
       - name: Setup Firefox and geckodriver
         uses: ./.github/actions/setup-firefox
       - name: Run server tests
+        env:
+          MOZ_HEADLESS: 1
         uses: ./.github/actions/bazel-test
         with:
           query: tests(//java/server/...) except attr(tags, 'lint|ie|edge|edgehtml|safari', tests(//java/server/...))