Tests: Disable visionOS cases because Xcode 15.0 excludes support

In commit 25977e177b (apple: add preliminary visionOS support, 2023-06-22)
we added visionOS based on a Xcode 15 beta version, but the Xcode 15.0
final release did not include it.  Disable the test cases until we can
improve the condition to detect availability.

Issue: #25266