[services] Tunnelbroker - Test of createdAt field in the messageItem

Summary: The new field createdAt was added at D3805. Adding the test for it to the messageItem fields testings.

Test Plan: Run yarn test-tunnelbroker-service all tests are run and succeed.

Reviewers: karol-bisztyga, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3806