MSTest - add a new option to have explicit Program.cs

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MSTest templates are using MSTest.Sdk and are currently relying on the auto-generated entry point. I think it's a good idea to make the entry point explicit as it is then easier for users to understand how they can plug into the test application.

My suggestion is to introduce a new option (CLI + VS) to have explicit entry point (vs auto-generated) and to have the explicit as default.

This requires MSTest 3.6 to be released.

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@mariam-abdulla Let me know if the task isn't clear enough so we could sync about it.