Update metal tests to only run on macOS

Author: rconnell9

Sources/_InternalTestSupport/MockBuildTestHelper.swift

@@ -20,6 +20,7 @@ import SPMBuildCore
 import TSCUtility
 
 public struct MockToolchain: PackageModel.Toolchain {
+    public let metalToolchainId: String?
     public let metalToolchainPath: Basics.AbsolutePath?
     #if os(Windows)
     public let librarianPath = AbsolutePath("/fake/path/to/link.exe")

Tests/BuildMetalTests/BuildMetalTests.swift

@@ -19,6 +19,7 @@ import Metal
 @Suite
 struct BuildMetalTests {
 
+#if os(macOS)
     @Test(
         .tags(
             .TestSize.large
@@ -65,5 +66,5 @@ struct BuildMetalTests {
             #expect(library.functionNames.contains("simpleVertexShader"))
         }
     }
-
+#endif
 }