test: fix flaky test

This commit fixes test-tls-set-secure-context.js. The test was
making one long lasting HTTP connection, followed by a number of
shorter lived connections. However, it was possible that the
connections were not received in the desired order. This commit
ensures that the long lasting connection is established before
making any other connections.

PR-URL: https://github.com/nodejs/node/pull/23811
Fixes: https://github.com/nodejs/node/issues/23807


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>