tests/net/lib/dns_addremove: Initialize resolver objects
tests/net/lib/dns_addremove: Initialize resolver objects

These were never being initialized by the test, leaving an embedded
k_mutex as bare memory.  That used to work (I guess?), it doesn't with
k_zync.

Signed-off-by: Andy Ross <61c3cfd4c0060414f91281a789685c974e47d1ec@google.com>

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