MM-63300: Fix flaky test TestBusySet (#31116)

(*Busy).Set would set its own timer, and additionally
send a message across the cluster. In this case, the cluster
is mocked locally. But the timer calculation happens again.

We marshal the expiry time with b.expires.Unix() and send that
as part of model.ServerBusyState. This is parsed again in
ClusterEventChanged and converted to duration with time.Until.

Therefore, if it takes longer for the code to reach those lines,
then the new time calculated would have already expired, failing
the test.

To fix this, we increase the timeout. This slows down the test
at the cost of extra reliability. This is a common failure point
with any timer related tests.

Additionally, we also change the condition in compareBusyState
to check for less-than rather than strict equality.

https://mattermost.atlassian.net/browse/MM-63300

```release-note
NONE
```

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/busy_test.go

@@ -16,7 +16,6 @@ import (
 )
 
 func TestBusySet(t *testing.T) {
-	t.Skip("https://mattermost.atlassian.net/browse/MM-63300")
 	cluster := &ClusterMock{Busy: &Busy{}, t: t}
 	busy := NewBusy(cluster)
 
@@ -26,11 +25,11 @@ func TestBusySet(t *testing.T) {
 
 	require.False(t, busy.IsBusy())
 
-	busy.Set(time.Millisecond * 500)
+	busy.Set(time.Second * 5)
 	require.True(t, busy.IsBusy())
 	require.True(t, compareBusyState(t, busy, cluster.Busy))
 
-	// should automatically expire after 500ms.
+	// should automatically expire after 5s.
 	require.Eventually(t, isNotBusy, time.Second*15, time.Millisecond*100)
 	// allow a moment for cluster to sync.
 	require.Eventually(t, func() bool { return compareBusyState(t, busy, cluster.Busy) }, time.Second*15, time.Millisecond*20)
@@ -105,13 +104,16 @@ func compareBusyState(t *testing.T, busy1 *Busy, busy2 *Busy) bool {
 	if busy1.IsBusy() != busy2.IsBusy() {
 		busy1JSON, _ := busy1.ToJSON()
 		busy2JSON, _ := busy2.ToJSON()
-		t.Logf("busy1:%s;  busy2:%s\n", busy1JSON, busy2JSON)
+		t.Logf("IsBusy() is not equal: busy1:%s;  busy2:%s\n", busy1JSON, busy2JSON)
 		return false
 	}
-	if busy1.Expires().Unix() != busy2.Expires().Unix() {
+	// busy2 is the cluster expiry which could potentially be later
+	// than busy1 because of the recalculation of new time at ClusterEventChanged
+	// and then again at setWithoutNotify.
+	if busy1.Expires().Unix() < busy2.Expires().Unix() {
 		busy1JSON, _ := busy1.ToJSON()
 		busy2JSON, _ := busy2.ToJSON()
-		t.Logf("busy1:%s;  busy2:%s\n", busy1JSON, busy2JSON)
+		t.Logf("busy1.Expires().Unix():%s is not less than busy2Expires().Unix():%s\n", busy1JSON, busy2JSON)
 		return false
 	}
 	return true