[PORT] add TestFlow.AssertNoReply and tests

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3125

To make sure the bot really ends.
Unexpected replies once happend in VA-Skill interactions in solutions project.

[stevengum]

@jwiley84 do you have bandwidth to take this on?

[jwiley84]

Yes.