It turns out that occasionally, the test thread ends up with its SynchronizationContext.Current set to the Windows Forms sync context. That's bad because nothing runs a message loop, so when AsyncSubject attempts to complete an awaited operation, it tries to do so by posting a message to a queue that's never going to be processed.
So we explicitly set the context to null for the test that expects that.
It turns out that occasionally, the test thread ends up with its SynchronizationContext.Current set to the Windows Forms sync context. That's bad because nothing runs a message loop, so when AsyncSubject attempts to complete an awaited operation, it tries to do so by posting a message to a queue that's never going to be processed.
So we explicitly set the context to null for the test that expects that.
Relates to (but doesn't fully deal with) #1885