tests: Fix eapol-key-auth memory use for the callback pointer struct

tests: Fix eapol-key-auth memory use for the callback pointer struct

This struct needs to remain valid through the lifetime of the
authenticator state machine, so move it to the context struct instead of
being a local stack variable inside the setup function that returns
before the actual test functionality is executed.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>