[sdk] Set MacOS version in tests using osx_sdk module

The platform.mac_version api was broken, and is now removed by
https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/5394455 .

Use the osx_sdk module's method of setting the MacOS version for tests
instead. This is how the tests of the osx_sdk module set the version.

Bug: https://issues.chromium.org/issues/331252586
Change-Id: I0a558e79303191d8a696ab0e7045a49519097a1a
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/361080
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
Auto-Submit: William Hesse <whesse@google.com>
3 files changed