tests: Fix clearing of IGTK PN in handshake retransmit test functions

tests: Fix clearing of IGTK PN in handshake retransmit test functions

When locating the position of the IGTK PN in the key data, we also need
to skip the KDE header, in addition to the keyid field. This fixes
hostapd RESEND_M3 and RESEND_GROUP_M1 behavior when PMF is negotiated
for the association. Previously, the IGTK KDE ended up getting
practically hidden since zeroing of the PN ended up clearing the KDE OUI
and Type fields.

Signed-off-by: Mathy Vanhoef <Mathy.Vanhoef@cs.kuleuven.be>