of: unittest: Add option string test case with longer path

There were regressions seen with commit 106937e8 ("of: fix handling
of '/' in options for of_find_node_by_path()"), where we couldn't handle
extra '/' before the ':'. Let's test for this now.

Confirmed that this test fails without the previous patch and passes
when patched. All other tests pass.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>