tests: net: dhcpv4: client: Verify Request xid

According to RFC 2131 Request message Exchange ID should be the same as
the one received in the Offer message from the server. Modify test to
verify that.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit 1c46508c54b6eb7d9ea78e5b4b4e16284cfa53bc)
1 file changed