netdevsim: add Makefile for selftests

Add a Makefile for netdevsim selftests and add selftests path to
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Signed-off-by: David Wei <dw@davidwei.uk>
Link: https://lore.kernel.org/r/20240130214620.3722189-5-dw@davidwei.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>