Fixes to timing tests that are failing intermittently under Appveyor

These are mostly adhoc fixes and may not stop all the errors but I'd like to get it merged and move on to something else for a while. We should watch for similar errors and come back to this if they recur.

Changes made:
  * Use stopwatch in DelayedConstraintTests.cs
  * Remove intermittently failing assert
  * Don't run timing tests in parallel
  * Fix logic error in NUnit.proj
Stopwatch.StartNew() would have saved a line of code here. I implemented it in the compatibility version.