scripts: dts: Add test for multiple binding directories

Add two bindings

    test-bindings/multidir.yaml
    test-bindings-2/multidir.yaml

and a new test-multidir.dts with two nodes that use them.

Verify that the two bindings were found by checking the
Device.binding_path attribute for the two device nodes.

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