Clear sync context on test that requires it to be absent #2026
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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