chore: migrate tests from mocha to vitest

Replace mocha 6 with vitest 4.1.11. done callbacks become awaited
promises with the assertions moved after the await; vitest.config.js
unsets PRERENDER_SERVICE_URL so the suite is hermetic, and runs test
files serially because both Express support apps bind the same port.
Travis now runs on Node 20, the minimum Vitest 4 supports.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: marceloprerender

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - "10"
+  - "20"
 script:
   - cd test/support/express3 && npm install
   - cd ../express4 && npm install