Occasional hang/segfault in mtl/metal tests

[christiangnrd]

https://buildkite.com/julialang/metal-dot-jl/builds/2540/steps/table?jid=019c7614-ae20-413d-9415-fa31fee624a6&tab=output

Potential culprit?

Metal.jl/lib/mtl/device.jl

Lines 17 to 20 in d1a8cc2

	function devices()
	list = NSArray(ccall(:MTLCopyAllDevices, id{NSArray}, ()))
	[reinterpret(MTLDevice, dev) for dev in list]
	end