Fix a flaky test that depends on real system time

[joelverhagen]

A test turned out to be flaky. Apparently Task.Delay can take less than the provided time to complete. I had assumed that the actual delay would always be greater than or equal to the provided TimeSpan, but apparently that is not true.

##[error]    NuGet.Services.AzureSearch.SearchService.AuxiliaryFileReloaderFacts+ReloadContinuouslyAsync.UsesReloadFrequencyOnSuccess [FAIL]

      Assert.InRange() Failure
      Range:  (00:00:00.1000000 - 01:00:00)
      Actual: 00:00:00.0975096
      Stack Trace:
        src\AzureSearchService\submodules\NuGet.Services.Metadata\tests\NuGet.Services.AzureSearch.Tests\SearchService\AuxiliaryFileReloaderFacts.cs(58,0): at NuGet.Services.AzureSearch.SearchService.AuxiliaryFileReloaderFacts.ReloadContinuouslyAsync.<UsesReloadFrequencyOnSuccess>d__3.MoveNext()
        --- End of stack trace from previous location where exception was thrown ---
           at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
           at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
        --- End of stack trace from previous location where exception was thrown ---
           at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
           at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
        --- End of stack trace from previous location where exception was thrown ---
           at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
           at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
      Output:
        | NuGet.Services.AzureSearch.SearchService.AuxiliaryFileReloader Information: Trying to reload the auxiliary data.
        | NuGet.Services.AzureSearch.SearchService.AuxiliaryFileReloader Information: Waiting 00:00:00.1000000 before attempting to reload the auxiliary data again.
        | NuGet.Services.AzureSearch.SearchService.AuxiliaryFileReloader Information: Trying to reload the auxiliary data.

Failed build:
https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=2706000

[agr]

I'm not sure ISystemTime is a good name but can't come up with a better name :)

[joelverhagen]

I'm not sure ISystemTime is a good name but can't come up with a better name :)

I drew inspiration from previous abstractions I've see around the DateTime/DateTimeOffset static methods. I figured we could add those here later when we need them and at that point it would be a generic wrapper around Time.