FanoutConfigResourceTest: assert configuration updates being visible

When updating a remote configuration using the FanoutConfigResource,
the updated values should be also visible in-memory and not simply
stored on the underlying filesystem.

Fix an issue where the updates to the remotes performed using
$GERRIT_SITE/etc/replication/remote.config fan-out config scheme
was persisted on disk but left a stale in-memory copy.

Change-Id: I2360ba447ffbc3695faad72e5674ca883f75081a