Fix default.metallib lookup under swift test with Swift Build

[heckj]

Summary

swift test's default build system (--build-system swiftbuild) nests resource bundles inside the .xctest and runs tests through the toolchain's swiftpm-testing-helper. The running binary is the toolchain helper, not the test bundle, so neither Bundle.main nor Bundle.allBundles names the .xctest, and the existing SwiftPM bundle search in load_swiftpm_library can never reach it. MLX aborts the entire test process on the first GPU call rather than failing one test.

fixes: ml-explore/mlx-swift#460

Reported downstream at ml-explore/mlx-swift#460, where a SwiftPM package with a test target hitting any MLX op takes down all 126 tests in the suite with:

MLX error: Failed to load the default metallib. library not found library not
found library not found library not found
  at .../Cmlx/mlx-c/mlx/c/array.cpp:232

Fix

In the Swift Build test layout, the metallib sits at a fixed offset from the binary: ../Resources/<bundle>.bundle/Contents/Resources/default.metallib. This adds that as one more colocated candidate, guarded by SWIFTPM_BUNDLE so it's a no-op for CMake/framework builds.

swift test --build-system native is unaffected -- its flat layout was already covered by the first colocated candidate -- but native is marked deprecated by SwiftPM.

Testing

Reproduced and fixed against a local project that consumes mlx-swift. I have a separate Swift package with a trait-gated target that depends on mlx-swift transitively (through mlx-swift-lm/MLXEmbedders), with its own swift-testing test target. On this machine's toolchain (Xcode 27.0, Swift 6.4.0.34.1):

• Unpatched (mlx-swift 0.31.6): swift test --traits MLXEmbedding with MLX_RUNTIME_TESTS=1 reproduces the exact reported failure:

  MLX error: Failed to load the default metallib. library not found library not found library not found library not found  at .../Cmlx/mlx-c/mlx/c/array.cpp:232
  

• Same command, with this patch applied: all 20 tests in that suite pass.

I also ran this against mlx-swift's own test suite (swift test --filter 'MLXTests.MLXArrayTests|MLXTests.StreamTests', 36 tests, real GPU evals) with the patch applied -- all pass, no regressions.