Fix more chat test failures

[peppy]

Ordering was not guaranteed because joins are asynchronous.

All test channel joines are confirmed before continuing now, guaranteeing order.

https://teamcity.ppy.sh/buildConfiguration/Osu_Build/833?hideProblemsFromDependencies=false&hideTestsFromDependencies=false&expandBuildTestsSection=true

TearDown : System.TimeoutException : "PM Channel 1 displayed" timed out
--TearDown
   at osu.Framework.Testing.Drawables.Steps.UntilStepButton.<>c__DisplayClass11_0.<.ctor>b__0()
   at osu.Framework.Testing.Drawables.Steps.StepButton.PerformStep(Boolean userTriggered)
   at osu.Framework.Testing.TestScene.runNextStep(Action onCompletion, Action`1 onError, Func`2 stopCondition)
--- End of stack trace from previous location ---
   at osu.Framework.Testing.TestSceneTestRunner.TestRunner.RunTestBlocking(TestScene test)
   at osu.Game.Tests.Visual.OsuTestScene.OsuTestSceneTestRunner.RunTestBlocking(TestScene test) in /opt/buildagent/work/ecd860037212ac52/osu.Game/Tests/Visual/OsuTestScene.cs:line 503
   at osu.Framework.Testing.TestScene.RunTestsFromNUnit()
------- Stdout: -------
[runtime] 2022-06-28 08:36:43 [verbose]: 💨 Class: TestSceneChatOverlay
[runtime] 2022-06-28 08:36:43 [verbose]: 🔶 Test:  TestKeyboardNextChannel
[runtime] 2022-06-28 08:36:43 [verbose]: Chat is now polling every 60000 ms
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #1 Setup request handler
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #2 Add test channels
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #3 Show overlay with channels
[runtime] 2022-06-28 08:36:43 [verbose]: Attempting to join public channel #channel-1
[runtime] 2022-06-28 08:36:43 [verbose]: Attempting to join public channel #channel-2
[runtime] 2022-06-28 08:36:43 [verbose]: Attempting to join PM channel test user 854
[runtime] 2022-06-28 08:36:43 [verbose]: Current channel changed to test user 854
[runtime] 2022-06-28 08:36:43 [verbose]: Attempting to join PM channel test user 270
[runtime] 2022-06-28 08:36:43 [verbose]: Attempting to join public channel #channel-9
[runtime] 2022-06-28 08:36:43 [verbose]: Joined public channel #channel-1
[runtime] 2022-06-28 08:36:43 [verbose]: Joined public channel #channel-2
[runtime] 2022-06-28 08:36:43 [verbose]: Unhandled Request Type: osu.Game.Online.API.Requests.CreateChannelRequest
[network] 2022-06-28 08:36:43 [verbose]: Failing request osu.Game.Online.API.Requests.CreateChannelRequest (System.InvalidOperationException: DummyAPIAccess cannot process this request.)
[runtime] 2022-06-28 08:36:43 [verbose]: Failed to join PM channel test user 854 (DummyAPIAccess cannot process this request.)
[runtime] 2022-06-28 08:36:43 [verbose]: Unhandled Request Type: osu.Game.Online.API.Requests.CreateChannelRequest
[network] 2022-06-28 08:36:43 [verbose]: Failing request osu.Game.Online.API.Requests.CreateChannelRequest (System.InvalidOperationException: DummyAPIAccess cannot process this request.)
[runtime] 2022-06-28 08:36:43 [verbose]: Failed to join PM channel test user 270 (DummyAPIAccess cannot process this request.)
[runtime] 2022-06-28 08:36:43 [verbose]: Joined public channel #channel-9
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #4 Select channel 1
[runtime] 2022-06-28 08:36:43 [verbose]: Current channel changed to #channel-1
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #5 Channel 1 is visible
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #6 Press document next keys
[runtime] 2022-06-28 08:36:43 [verbose]: Current channel changed to #channel-2
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #7 Channel 2 is visible
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #8 Press document next keys
[runtime] 2022-06-28 08:36:43 [verbose]: Current channel changed to #channel-9
[runtime] 2022-06-28 08:36:43 [verbose]: 🔸 Step #9 PM Channel 1 displayed
[network] 2022-06-28 08:36:44 [verbose]: Request to https://a.ppy.sh/854 failed with System.Net.WebException: NotFound.
[network] 2022-06-28 08:36:44 [verbose]: Request to https://a.ppy.sh/270 failed with System.Net.WebException: NotFound.
[runtime] 2022-06-28 08:36:53 [verbose]: 💥 Failed (on attempt 7,820)
[runtime] 2022-06-28 08:36:53 [verbose]: ⏳ Currently loading components (0)
[runtime] 2022-06-28 08:36:53 [verbose]: 🧵 Task schedulers
[runtime] 2022-06-28 08:36:53 [verbose]: LoadComponentsAsync (standard) concurrency:4 running:0 pending:0
[runtime] 2022-06-28 08:36:53 [verbose]: LoadComponentsAsync (long load) concurrency:4 running:0 pending:0
[runtime] 2022-06-28 08:36:53 [verbose]: 🎱 Thread pool
[runtime] 2022-06-28 08:36:53 [verbose]: worker:          min 32     max 32,767 available 32,766
[runtime] 2022-06-28 08:36:53 [verbose]: completion:      min 32     max 1,000  available 1,000
[runtime] 2022-06-28 08:36:53 [debug]: Focus on "ChatTextBox" no longer valid as a result of unfocusIfNoLongerValid.
[runtime] 2022-06-28 08:36:53 [debug]: Focus changed from ChatTextBox to nothing.