of: unittest: add overlay gpio test to catch gpio hog problem

of: unittest: add overlay gpio test to catch gpio hog problem

Geert reports that gpio hog nodes are not properly processed when
the gpio hog node is added via an overlay reply and provides an
RFC patch to fix the problem [1].

Add a unittest that shows the problem.  Unittest will report "1 failed"
test before applying Geert's RFC patch and "0 failed" after applying
Geert's RFC patch.

[1] https://lore.kernel.org/linux-devicetree/20191230133852.5890-1-geert+renesas@glider.be/

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>