Fix occasional failure in realm test proceedings due to incorrect `Debug.Assert`
Fix occasional failure in realm test proceedings due to incorrect `Debug.Assert`

After a `BlockAllOperations`, the restoration of the `updateRealm`
instance is not instance. It is posted to a `SynchronizationContext`.
The assertion which has been removed in this commit was assuming it
would always be an immediate operation.

To ensure this works as expected, I've tracked the initialised state via
a new `bool`.

```csharp
System.TimeoutException : Attempting to block for migration took too long.

  1) Host threw exception System.AggregateException: One or more errors occurred. (: )
 ---> NUnit.Framework.AssertionException: :
   at osu.Framework.Logging.ThrowingTraceListener.Fail(String message1, String message2)
   at System.Diagnostics.TraceInternal.Fail(String message, String detailMessage)
   at System.Diagnostics.TraceInternal.TraceProvider.Fail(String message, String detailMessage)
   at System.Diagnostics.Debug.Fail(String message, String detailMessage)
   at osu.Game.Database.RealmAccess.BlockAllOperations() in /opt/buildagent/work/ecd860037212ac52/osu.Game/Database/RealmAccess.cs:line 813
   at osu.Game.OsuGameBase.<>c__DisplayClass108_1.<Migrate>b__0() in /opt/buildagent/work/ecd860037212ac52/osu.Game/OsuGameBase.cs:line 449
   at osu.Framework.Threading.ScheduledDelegate.RunTaskInternal()
   at osu.Framework.Threading.Scheduler.Update()
   at osu.Framework.Graphics.Drawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
```

https://teamcity.ppy.sh/buildConfiguration/Osu_Build/322?hideProblemsFromDependencies=false&hideTestsFromDependencies=false&expandBuildTestsSection=true

ppy/osu,peppy/osu,peppy/osu,ppy/osu,peppy/osu,ppy/osu