of/selftest: Fix testing when /aliases is missing

The /aliases node isn't always present in the device tree, but the
unittest code assumes that /aliases is there. Add a check when inserting
the testcase data to see if of_aliases needs to be updated, and undo the
settings when the nodes are removed.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Gaurav Minocha <gaurav.minocha.os@gmail.com>
Cc: <stable@vger.kernel.org>