Add graceful stop specs

[Aaronontheweb]

Changes

Adding some unit tests for GracefulStop since apparently we've never had any dedicated tests for this in the test suite. This is in preparation for some changes I'm going to introduce in #6102

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.

[Aaronontheweb]

Marked this test as Skipped for now - going to change that in #6102

This API is wrong. We shouldn't throw an Exception if graceful stop fails to complete on-time - we should simply return false. Otherwise there's no point to having a return value at all.

[Aaronontheweb]

All of the tests passed but the .NET test runner hit the deck:

Written 699301888 bytes (170728 pages) to core file
The active test run was aborted. Reason: Test host process crashed
Data collector 'Blame' message: Could not start process dump: System.PlatformNotSupportedException: Unsupported operating system: Linux 5.4.0-1086-azure #91~18.04.1-Ubuntu SMP Thu Jun 23 20:33:05 UTC 2022, and framework: .NETCoreApp,Version=v3.1.
   at Microsoft.TestPlatform.Extensions.BlameDataCollector.CrashDumperFactory.Create(String targetFramework)
   at Microsoft.TestPlatform.Extensions.BlameDataCollector.ProcessDumpUtility.CrashDump(Int32 processId, String tempDirectory, DumpTypeOption dumpType, String targetFramework, Boolean collectAlways)
   at Microsoft.TestPlatform.Extensions.BlameDataCollector.ProcessDumpUtility.StartTriggerBasedProcessDump(Int32 processId, String testResultsDirectory, Boolean isFullDump, String targetFramework, Boolean collectAlways)
   at Microsoft.TestPlatform.Extensions.BlameDataCollector.BlameCollector.TestHostLaunchedHandler(Object sender, TestHostLaunchedEventArgs args).
Data collector 'Blame' message: The specified inactivity time of 30 seconds has elapsed. Collecting hang dumps from testhost and its child processes.
Results File: /home/vsts/work/1/s/TestResults/_fv-az376-119_2022-09-17_16_45_43.trx