Add the qa spec for bp:more-selectable-swift-tests

Add the spec which describes why and how to extend selectability of Swift
tests.

Partially implements blueprint more-selectable-swift-tests

Change-Id: Iea2d4c6f0663528d3e8ed2a9a2ed1defaddb2758
Merge "Add the qa spec for bp:more-selectable-swift-tests"