test: fix test-tls-server-verify.js on Windows CI

The test runs two test cases at a time. This is not relevant to what the
test is actually testing. Not sure why doing it that way causes a
deadlock on some Windows servers, but running one at a time fixes it.

Fixes: https://github.com/nodejs/node/issues/18269

PR-URL: https://github.com/nodejs/node/pull/18382


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>