Removes OF_UNITTEST dependency on OF_DYNAMIC config symbol

This patch intends to remove the unittests dependency on
the functions defined in dynamic.c. So, rather than calling
of_attach_node defined in dynamic.c, minimal functionality
required to attach a new node is re-defined in unittest.c.
Also, now after executing the tests the test data is not
removed from the device tree so there is no need to call
of_detach_node.

Tested with and without OF_DYNAMIC enabled on ppc, arm and
x86

Signed-off-by: Gaurav Minocha <gaurav.minocha.os@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>