Enabled scheduler, categorized three known test cases as failure

Background
-----
This pull request enables the scheduler to assess the impact it has on unit test cases, especially after @ikeough [fixed issues found in Revit tests](https://github.com/DynamoDS/Dynamo/pull/2684) when scheduler is enabled.

The scheduler is enabled through a compiler flag ```ENABLE_DYNAMO_SCHEDULER``` defined in the following projects:

    src/DynamoCore/DynamoCore.csproj
    src/DynamoRevit/DynamoRevit.csproj
    src/Libraries/DynamoWatch3D/Watch3D.csproj
    src/Libraries/Revit/RevitServices/RevitServices.csproj


Unit Tests
-----
It also disables the follow two unit test cases that are known to fail due to scheduler changes:

    RecordedTestsDSEngine.TestCancelExecution
    RecordedTestsDSEngine.TestCancelExecutionWhileLoop
    ExecutionEventsObserver.TestPreAndPostExec


These test cases fail due to the fact that task cancellation (for long running evaluation) is only planned to be [implemented in subsequent sprints](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4994). As for the ```TestPreAndPostExec```, it is due to the fact that ```GraphPreExecution``` and ```GraphPostExecution``` events are no longer sent when ```DynamoScheduler``` is enabled (@lukechurch, I will be working on this [as a separate task](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5023) right after this).

Just for your information, guys. @kronz, @lukechurch, @ikeough, @RodRecker, @sharadkjaiswal.