[clang-format] Set of unit test to begin to validate that we don't change defaults

Summary:
This revision is to complement {D75791} so we can be sure that we don't change any default behavior.

For now just add rules to cover AfterExternBlock, but in the future we should add cases to cover the other BraceWrapping rules for each style. This will help guard us when we change code inside of the various getXXXStyle() functions to ensure we are not breaking everyone.

Reviewed By: MarcusJohnson91

Subscribers: cfe-commits

Tags: #clang, #clang-format

Differential Revision: https: