[Microsoft.Build] Make BuildSubmissionTest.EndBuildWaitsForCompletion more reliable...

[Microsoft.Build] Make BuildSubmissionTest.EndBuildWaitsForCompletion more reliable (#5209)

We've seen this test failure a few times:

```
MESSAGE:
  #2
  Expected: greater than or equal to 00:00:01
  But was:  00:00:00.9544900

+++++++++++++++++++
STACK TRACE:
  at MonoTests.Microsoft.Build.Execution.BuildSubmissionTest.EndBuildWaitsForSubmissionCompletion () [0x000d8] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-amd64/mcs/class/Microsoft.Build/Test/Microsoft.Build.Execution/BuildSubmissionTest.cs:110
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00032] in /mnt/jenkins/workspace/test-mono-mainline-linux/label/ubuntu-1404-amd64/mcs/class/corlib/System.Reflection/MonoMethod.cs:305
```

Rewriting the test to use Stopwatch instead of DateTime
for measuring elapsed time should make it more reliable.