[2.4.0] Checkmk Werk 16798 created: ups_test: Handle unknown time since last test

[//]: # (werk v2) # ups_test: Handle unknown time since last test key | value ---------- | --- date | 2024-06-17T05:43:13+00:00 version | 2.4.0b1 class | fix edition | cre component | checks level | 1 compatible | yes The time since the last battery test is calculated via the difference between the current uptime and the uptime when the test was initiated. This means the time cannot be determined if the uptime of the test initiation is larger than the current uptime. Before, this lead to a ``` ValueError: Cannot render negative timespan ``` Now, the check will become UNKNOWN instead.