Also filter <build-dir>/generated_compilation from testing filesets.

Like generated_tests, this is a scratch directory created by package:test_runner.

Change-Id: If26f77dbed50d53b2551d1367efded143c18f0e0
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/508401
Commit-Queue: Ivan Inozemtsev <iinozemtsev@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>