of: unittest: allow base devicetree to have symbol metadata

The overlay metadata nodes in the FDT created from testcases.dts
are not handled properly.

The __fixups__ and __local_fixups__ node were added to the live
devicetree, but should not be.

Only the first property in the /__symbols__ node was added to the
live devicetree if the live devicetree already contained a
/__symbols node.  All of the node's properties must be added.

Tested-by: Alan Tull <atull@kernel.org>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>