tests: fix error message wording due to commit eb96eb68

Change-Id: I18850326f3a4b1c3a481356735facf8d4ac271c1
system status now contains complete user objects instead of usernames

Commit 90781cda actually made this change for the 'current_loan' key
already (due to the change in System.get_loan_details). This patch
updates the 'current_reservation' key to match, and updates the client
to handle the new object in both keys.

Change-Id: I359493e8ec13e6e9eb6c8cb3981fd108a52d060f