Improve Metal.jl tests

- Simplify test to directly check that lu_instance returns an LU type
- Make Metal tests only run on macOS using Sys.isapple() check
- Add LinearAlgebra import for LU type check

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ChrisRackauckas

test/gpu/metal.jl

@@ -1,7 +1,8 @@
 using Metal
 using ArrayInterface
+using LinearAlgebra
 
 using Test
 
-# Test whether lu_instance throws an error when invoked with a Metal.jl gpu array
-@test !isa(try ArrayInterface.lu_instance(MtlArray([1.f0 1.f0; 1.f0 1.f0])) catch ex ex end, Exception) 
+# Test that lu_instance works with Metal.jl gpu arrays
+@test isa(ArrayInterface.lu_instance(MtlArray([1.f0 1.f0; 1.f0 1.f0])), LU) 

test/runtests.jl

@@ -21,6 +21,8 @@ end
     if GROUP == "GPU"
         activate_gpu_env()
         @time @safetestset "CUDA" begin include("gpu/cuda.jl") end
-        @time @safetestset "Metal" begin include("gpu/metal.jl") end
+        if Sys.isapple()
+            @time @safetestset "Metal" begin include("gpu/metal.jl") end
+        end
     end
 end