Fix mutable_span_bounds_check_tests.swift on iOS: REQUIRES: OS=macosx

[atrick]

This test fails on iOS and blocks PR testing:

swift-PR-macos/branch-main/swift/test/SILOptimizer/Inputs/SpanExtras.swift:172:11: error: 'Span' is only available in iOS 9999 or newer
 170 | 
 171 | @available(macOS 9999, *)
 172 | extension Span where Element: ~Copyable {
     | |         `- error: 'Span' is only available in iOS 9999 or newer
     | `- note: add @available attribute to enclosing extension

[atrick]

@swift-ci test

[meg-gupta]

Thanks!