tests: Clear AP scan cache after primary BSS on channel 40

tests: Clear AP scan cache after primary BSS on channel 40

A number of test cases using 40 MHz or wider channels with the primary
channel 36 were failing when executed after dpp_chirp_ap_5g since that
test case was running an AP on the channel 40 and resulting in need to
swap the primary and the secondary channels in the following test case.
Fix this by clearing the AP scan cache explicitly for such cases.

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