scripts: edtlib: Add backwards compatibility for 'category:'

Having backwards compatibility for !include and 'constraint:' is silly
without also having backwards compatibility for 'category:', because
that forces a binding change anyway.

Add backwards compatibility for 'category:', and just print a
deprecation warning when it's used.

Also move tests for deprecated features into a dedicated
test-bindings/deprecated.yaml binding, instead of piggybacking on other
tests.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>