Fix panic in TestPartitionOffsetWatcher_Concurrency

[pracucci]

What this PR does

Earlier today I merged #6982, which introduces the test TestPartitionOffsetWatcher_Concurrency. The test panics if lastNotifiedOffset == lastWatchedOffset because rand.In63n(0) is not a valid input value:

panic: invalid argument to Int63n

goroutine 1684 [running]:
math/rand.(*Rand).Int63n(0xc000447470, 0x0)
	/usr/local/go/src/math/rand/rand.go:122 +0x125
math/rand.Int63n(0xc0007a4960?)
	/usr/local/go/src/math/rand/rand.go:443 +0x2f
github.com/grafana/mimir/pkg/storage/ingest.TestPartitionOffsetWatcher_Concurrency.func2()
	/__w/mimir/mimir/pkg/storage/ingest/partition_offset_watcher_test.go:231 +0xb8
github.com/grafana/mimir/pkg/storage/ingest.runAsync.func1()
	/__w/mimir/mimir/pkg/storage/ingest/writer_test.go:354 +0x82
created by github.com/grafana/mimir/pkg/storage/ingest.runAsync in goroutine 675
	/__w/mimir/mimir/pkg/storage/ingest/writer_test.go:352 +0xd1

Thanks @dimitarvdimitrov for reporting it.

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[dimitarvdimitrov]

thanks!