Ensure the mock keychain trait is only available on supported platforms

Author: AlexNachbaur

Tests/TestCommon/Traits/MockKeychainTrait.swift

@@ -10,6 +10,7 @@
 // See the License for the specific language governing permissions and limitations under the License.
 //
 
+#if os(iOS) || os(macOS) || os(tvOS) || os(watchOS) || (swift(>=5.10) && os(visionOS))
 import Testing
 import Foundation
 
@@ -42,3 +43,4 @@ extension Test {
 extension Trait where Self == MockKeychainTrait {
     static var mockKeychain: Self { Self() }
 }
+#endif