[fxtest] Add --run-affected-tests for developers to run all affected tests more easily

We use the output from the --show-affected-tests as the input
of `fx add-test`, then after adding tests, we then append
the affected tests label into flag.selection
to be integrated with the original fx test flow

Fixed: 495199149
Change-Id: Ibcc4f2b5322896b6dabc92506041de7d5a9e7ebc
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1549721
Reviewed-by: Christopher Johnson <crjohns@google.com>
Commit-Queue: michael liao <weijliao@google.com>
Fuchsia-Auto-Submit: michael liao <weijliao@google.com>