Fix sporadic failures of TestRunnerJobRequestMessageFromRunService_AuthMigrationFallback

[theihor]

I ran into an issue where TestRunnerJobRequestMessageFromRunService_AuthMigrationFallback would fail with the following error:

#25 360.8   Starting test execution, please wait...
#25 362.1   A total of 1 test files matched the specified pattern.
#25 460.2   [xUnit.net 00:01:27.31]     GitHub.Runner.Common.Tests.Listener.RunnerL0.TestRunnerJobRequestMessageFromRunService_AuthMigrationFallback [FAIL]
#25 461.2     Failed GitHub.Runner.Common.Tests.Listener.RunnerL0.TestRunnerJobRequestMessageFromRunService_AuthMigrationFallback [177 ms]
#25 461.4 EXEC : error Message:  [/opt/runner/src/dir.proj]
#25 461.4      Moq.MockException : 
#25 461.4   Expected invocation on the mock exactly 2 times, but was 3 times: x => x.LoadCredentials(True)
#25 461.4   
#25 461.4   Performed invocations:
#25 461.4   
#25 461.4      Mock<ICredentialManager:33> (x):
#25 461.4   
#25 461.4         IRunnerService.Initialize(TestHostContext)
#25 461.4         IRunnerService.Initialize(TestHostContext)
#25 461.4         ICredentialManager.LoadCredentials(True)
#25 461.4         ICredentialManager.LoadCredentials(False)
#25 461.4         ICredentialManager.LoadCredentials(True)
#25 461.4         IRunnerService.Initialize(TestHostContext)
#25 461.4         ICredentialManager.LoadCredentials(True)
#25 461.4   
#25 461.4     Stack Trace:
#25 461.4        at Moq.Mock.Verify(Mock mock, LambdaExpression expression, Times times, String failMessage) in /_/src/Moq/Mock.cs:line 332
#25 461.4      at Moq.Mock`1[[GitHub.Runner.Listener.Configuration.ICredentialManager, Runner.Listener, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].Verify[VssCredentials](Expression`1 expression, Times times) in /_/src/Moq/Mock`1.cs:line 825
#25 461.4      at GitHub.Runner.Common.Tests.Listener.RunnerL0.TestRunnerJobRequestMessageFromRunService_AuthMigrationFallback() in /opt/runner/src/Test/L0/Listener/RunnerL0.cs:line 981
#25 461.4   --- End of stack trace from previous location ---
#25 463.1   Results File: /opt/runner/src/Test/TestResults/_buildkitsandbox_2025-05-27_00_18_24.trx
#25 463.1   
#25 463.2   Failed!  - Failed:     1, Passed:   682, Skipped:     0, Total:   683, Duration: 1 m 21 s - Test.dll (net8.0)
#25 464.3 /opt/runner/src/dir.proj(60,9): error MSB3073: The command "dotnet test Test/Test.csproj -c Debug --no-build --logger:trx" exited with code 1.
#25 464.8 Failed: failed tests

As far as I was able to determine, this happens because it's executed in emulation (it's part of docker image build for various platforms) which makes it slow, while this test depends on timing of particular events happening.

Suggested change might not be the right fix: I'm making a guess that additional calls to LoadCredentials are ok in the context of this test.

@TingluoHuang Please take a look, thanks.