test_observer requires macOS 13.0

[keith]

When updating rules_swift on swift-syntax I noticed that test_observer gets automatically added and requires macOS 13.0 based on some of the APIs it's using. swift-syntax currently pins in SwiftPM to 10.15, so I was trying to keep that matched in bazel.

Seems to me like we can do a few things:

1. Force a minimum macOS version supported by rules_swift
2. Change test_observer to support older macOS versions
3. Disable test_obeserver if we're targeting something lower than 13.0

thoughts?