Specify the platform version as the fixtures depend on swift concurrency

[rconnell9]

Specify the platform version as the fixtures depend on swift concurrency.

Motivation:

Tests failing locally due to code in the fixtures depending on Swift Concurrency features.

Modifications:

Specify the platform version in the fixtures.

Result:

Tests pass

[rconnell9]

@swift-ci test

[bkhouri]

The macOS self hosted pipeline failed as a XCTest invocation crashed

13:35:34  Test Case '-[BasicsTests.AsyncProcessTests testAsyncStreamHighLevelAPI]' started.
13:35:34  
13:35:34  objc[42751]: Class DTXDecompressionException is implemented in both /System/Library/PrivateFrameworks/DTXConnectionServices.framework/Versions/A/DTXConnectionServices (0x7ff85c4691d8) and /Applications/Xcode.app/Contents/SharedFrameworks/DTXConnectionServices.framework/Versions/A/DTXConnectionServices (0x102532830). One of the two will be used. Which one is undefined.
<...SNIP...>

Let's retriever the macOS builds

@swift-ci test macOS

[rconnell9]

@swift-ci test macOS