Remove use of step.defer_results from test results step

Part of migrating uses of step.defer_results to the new
recipe module defer.

Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1495428
Change-Id: I957f163c91f7c34c30439b18a8fccfdf980dace8
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/334242
Commit-Queue: William Hesse <whesse@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>