[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