Replace a bunch of references to .packages

Most of these arguments are flowing through to a `--packages` argument
which allows passing the `package_config.json` file. In the long term we
should remove the `.packages` file entirely.

TEST=None, the change should have no visible impact.

Change-Id: I60a8b175d5e217a85588bbcb91cc5095514066b4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/211503
Auto-Submit: Nate Bosch <nbosch@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>
Reviewed-by: Kevin Moore <kevmoo@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

pkg/testing/testing.json

@@ -3,7 +3,7 @@
 "":"for details. All rights reserved. Use of this source code is governed by a",
 "":"BSD-style license that can be found in the LICENSE.md file.",
 
-  "packages": "../../.packages",
+  "packages": "../../.dart_tool/package_config.json",
 
   "suites": [
     {