tests: net: mld: fix waiting for MLDv2 report after query

This commit fixes 'test_verify_send_report()' function as it was
not properly waiting for a MLDv2 report after sending a query. The
asserted value was set for the previous report sent after joining a
group.

Signed-off-by: Konrad Derda <konrad.derda@nordicsemi.no>