Message when old tests are reused

[mattias-p]

It should clear from a result page whether a new test request with identical parameter would start a new test or reuse an old result. If an old result would be reused, it should also be clear how long the user would have to wait before a new test can be started.

Dependencies:

• Expose age_reuse_previous_test via RPCAPI zonemaster-backend#1255
• The start time of a test result should be exposed by the RPCAPI. N.b., the start time is the timestamp the reuse determination is currently based on. N.b., this will be provided by Formal states in tables zonemaster-backend#1246.