Ability to test OAuthPrompt and mock OAuth APIs

[Jeffders]

Description

Ability to mock the OAuth APIs on a BotAdapter
Implementation of mock in TestAdapter
Ensure OAuthPrompt uses the new interface
Sample unit tests for how to test OAuth APIs and the OAuthPrompt

Testing

Yes, tests on TestAdapter and OAuthPrompt

[coveralls]

Pull Request Test Coverage Report for Build #1934

• 42 of 47 (89.36%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.8%) to 84.184%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/botbuilder-dialogs/src/prompts/oauthPrompt.ts	2	3	66.67%
libraries/botbuilder-core/src/testAdapter.ts	38	40	95.0%
libraries/botbuilder/src/botFrameworkAdapter.ts	2	4	50.0%

Totals
Change from base Build #1932:	1.8%
Covered Lines:	2824
Relevant Lines:	3232

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💛 - Coveralls