util&mesa: add and use os_file_create_unique()

Merged Eric Engestrom requested to merge eric/mesa:mesa-shader_test-race into master

Better fix for !1085 (closed)

This is untested though, just like !1085 (closed), but this time the code is not shuffled around, so I would appreciate if someone could tell me how to test this, and if someone with a windows machine could test it for me :)

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