statuseshistory pruning test failure on s390 and arm64

[Tim Penhey]

FAIL: status_history_test.go:29: StatusHistorySuite.TestPruneStatusHistoryBySize

[LOG] 0:01.148 INFO juju.state statuseshistory size pruning finished: 9000 rows deleted
status_history_test.go:52:
    // When writing this test, the size was 6670 for about 0,00015 MB per entry
    // but that is a size that can most likely change so I wont risk a flaky test
    // here, enough to say that if this size suddenly is no longer less than
    // half its good reason for suspicion.
    c.Assert(historyLen, jc.LessThan, 10000)
... obtained int = 11006
... expected int = 10000

Can be seen from jenkins s390x and arm64 unit test runs.

[Tim Penhey (thumper)]

https://github.com/juju/juju/pull/8419