Tests that fail in worker threads may crash testing process

There are some tests that use `ThreadPool.QueueUserWorkItem` to run code on a worker thread, and then have test assertions that run on that thread. Some of these are in `BuildManager_Tests`, and there may be others elsewhere.

If these assertions on the worker threads fail, then the test process can crash without correctly reporting test results. This can make it hard to even figure out which tests are causing the issue.

With changes to support #2706, the way the failure manifests is with the following in the test log (which is the redirected output of the test process):

```
System.Runtime.Serialization.SerializationException: Type 'Xunit.Sdk.NotEqualException' in Assembly 'xunit.assert, Version=2.3.0.3820, Culture=neutral, PublicKeyToken=8d05b1bb7a6fdb6c' is not marked as serializable.
at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder)
at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.Serialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder)
at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object graph, Header[] inHeaders, __BinaryWriter serWriter, Boolean fCheck)
at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph, Header[] headers, Boolean fCheck)
at System.Runtime.Remoting.Channels.CrossAppDomainSerializer.SerializeObject(Object obj, MemoryStream stm)
at System.AppDomain.Serialize(Object o)
at System.AppDomain.MarshalObject(Object o)
```