ConcurrentJobConfigurationShouldBeRespected is a bit flakky

[nulltoken]

Describe the bug
As discussed in #166 (comment)

[xUnit.net 00:00:01.19]            at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
[xUnit.net 00:00:01.19]            at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task, ConfigureAwaitOptions options)
  Passed NCronJob.Tests.NCronJobIntegrationTests.TriggeringInstantJobWithoutRegisteringContinuesToWork [2 ms]
  Passed NCronJob.Tests.GlobalExceptionHandlerTests.ShouldStopProcessingWhenHandlerReturnsTrue [289 ms]
  Passed NCronJob.Tests.JobOptionBuilderTests.ShouldCreateJobOptionsWithCronExpression [1 ms]
  Passed NCronJob.Tests.JobOptionBuilderTests.ShouldAddMultipleCronJobsEvenWithoutParameters [< 1 ms]
  Passed NCronJob.Tests.JobOptionBuilderTests.ShouldCreateMultipleJobsWithoutAnd [< 1 ms]
  Passed NCronJob.Tests.JobOptionBuilderTests.ShouldCreateMultipleJobsWithParameters [< 1 ms]
  Passed NCronJob.Tests.RuntimeJobRegistryTests.TryRegisterShouldIndicateSuccess [9 ms]
  Failed NCronJob.Tests.NCronJobIntegrationTests.ConcurrentJobConfigurationShouldBeRespected [269 ms]
  Error Message:
   Shouldly.ShouldAssertException : await WaitForJobsOrTimeout(1, TimeSpan.FromMilliseconds(50))
    should be
False
    but was
True
  Stack Trace:
     at NCronJob.Tests.NCronJobIntegrationTests.ConcurrentJobConfigurationShouldBeRespected() in /_/tests/NCronJob.Tests/NCronJobIntegrationTests.cs:line 359
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task, ConfigureAwaitOptions options)
--- End of stack trace from previous location ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task, ConfigureAwaitOptions options)
  Passed NCronJob.Tests.JobRunStatesTests.CompletedJobRunsCannotChangeTheirStateFurther(value: Completed) [71 ms]
  Passed NCronJob.Tests.JobRunStatesTests.CompletedJobRunsCannotChangeTheirStateFurther(value: Cancelled) [< 1 ms]
  Passed NCronJob.Tests.JobRunStatesTests.CompletedJobRunsCannotChangeTheirStateFurther(value: Skipped) [< 1 ms]