test: check for session.post() errors in test-insepctor-context

If session.post() generates an error, it is currently ignored. Add check
for error by adding a callback to session.post() invocation.

PR-URL: https://github.com/nodejs/node/pull/30649


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>