helper/schema: Fix setting a set in a list caused error

[faridnsh]

The added test in this commit, without the fix, will make d.Set return
the following error:

Invalid address to set: []string{"ports", "0", "set"}

This was due to the fact that setSet in feild_writer_map tried to
convert a slice into a set by creating a temp set schema and calling
writeField on that with the address([]string{"ports", "0", "set"}" in
this case). However the temp schema was only for the set and not the
whole schema as seen in the address so, it should have been []string{"set"}"
so it would align with the schema.

This commits adds another variable there(tempAddr) which will only
contain the last entry of the address that would be the set key, which
would match the created schema

This commit potentially fixes the problem described in #16331

[jbardin]

This looks good, thanks for catching this @alFReD-NSH!

[ghost]

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