tests: Disconnect after OCV tests showing valid OCI

tests: Disconnect after OCV tests showing valid OCI

These test cases that used external EAPOL handling to test hostapd
behavior left the STA in associated state at the end and that could
result in race conditions showing up as a new association related event
in the following test case. Minimize risk of that resulting in test case
failures by explicit disconnecting at the end of the test case.

This was found with this test case sequence:
autogo_many wpa2_ocv_ap_unexpected1 wpa2_ocv_sta_override_eapol

Signed-off-by: Jouni Malinen <quic_jouni@quicinc.com>