[Synthetics UI] Track state for last test run separatedly from the test run table

[afgomez]

In the redux store we keep the information for the test runs in monitorDetails.pings. This is used for both the test run table, and the latest test run block. We treat the first ping as the last test run.

In #144029 we added pagination to the test run table. The pagination overwrites the contents of the store in monitorDetails.pings when the user switches pages. That means that for any page other than the first one, the first element in the monitorDetails.pings array is no longer the latest test run. This was commented as a known issue in the linked PR.

After speaking with @shahzad31 the best solution would be to keep the state for the last test run block separate from the list of pings. This is the issue to implement such change.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)